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NEW TOPIC: Increased Air Pressure on the Front ..

T

Triggerfifty

Sergeant
Full Member
Minuteman
Jun 1, 2005
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Long range guys, new discussion subject.

Roughly stated, winds that run from your 6 o'clock to the 12 o'clock downrange. The terrain PAST maximum ordinate rises sharply with 45 degrees either side of the gun target line. Phone below is a good example.

EVENT: Rounds string up and down the hillside in a footprint about 1 MOA wide, but 5 or more MOA long, (stringing up and down).

For the sake of discussion, name some factors that might cause this stringing. I'm talking more about exterior ballistic factors here, but feel free to include shooter/gun influences, gun design, etc.

You can see the aiming target 8' X 12' white square down range, which is on the up slope of the ground from the gun. Range to target is 1500 - 1800 yards. Gun of course is the 408 CheyTac.

DSC01295.JPG


In this situation, the target is located just above the windmeter in the observer's left hand. Shooting uphill, downhill, or flat and level? What are the potential errors that can occur here. What gun influences can influence the placement of the shots? The picture below is of the gun position, based on what you see, what are the potential gun errors? What might the shooter / observer do to better support the position, (non-tactical)?

Food for thought and discussion. Remember, nothing is what it appears to be. Deny everything, admit nothing and make counteraccusations.

DSC00051.JPG


DSC00056.JPG
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

In Reference: (natural updraft and or downdraft at hill, mountain or cliff)

In the ball park, what velocity is a draft? What is it's effect? Is it predictable? There is a specific event that happens in these conditions, downhill winds are not a problem. It's not the horizontal component of windage (relatively speaking).

Is stringing progressive or random?

Stringing is based on current air pressures, surface tension drag factors (smooth vs., very rocky ground), stringing appears random, BUT there are connecting and identifiable factors that can be managed.

Like zen stuff, a question is responded with deeper and deeper questions. This is solvable.

Trigger
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

WOW, man I am glad you back around!!

First discussion subject on what can cause the vertical stringing in the pic:

1. Range of 1500-1800 is way to much leadway that will cause that much vertical. example if target is 1600 and you calculating solution for 1800 then you are 10moa high and vice versa.

2. It appears that the wind coming from shooters back and going toward the target can actually "pool" up in the area of the target which could cause a turbulence wind pattern.

3. If angle of target from center scope was not taken into account.

4. Weapon system is laying in out broad direct sunlight. How many rounds fired and timeframe fired in could account for some elevation deviation due to "preheating" the round in chamber which will cause the MV to vary.

Second discussion example:
1. I am going to go with target is "barely" downhill.
2. Potential gun errors??? Has me stumped!!


Cannot wait to get your answers!!

Thanks
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

First discussion subject on what can cause the vertical stringing in the pic:

1. Range of 1500-1800 is way to much leadway that will cause that much vertical. example if target is 1600 and you calculating solution for 1800 then you are 10moa high and vice versa.

<span style="font-weight: bold"> </span> <span style="font-style: italic"> </span> We're assuming that the distance is precisely determined. Assuming also that the gun and shooter are capable of 1 MOA at this range under stable air conditions.

2. It appears that the wind coming from shooters back and going toward the target can actually "pool" up in the area of the target which could cause a turbulence wind pattern.

<span style="font-weight: bold"> </span> <span style="font-style: italic"> </span> That is in the ball park, but it's not pooling or swirling there. It is a factor that causes vertical stringing.

3. If angle of target from center scope was not taken into account.

<span style="font-weight: bold"> </span> <span style="font-style: italic"> </span> Yet another factor. In the first photo, the guns has an up elevation, in the second, the gun is depressed down, but it's the gun's mechanical setup that is one of the greatest potentials here. Between the problem with the guns layout (HINT: due to the terrain it's sitting on), and the problem of a tailwind driving into the face of the hill, the situation is almost uncontrollable, but can be managed, provided the target is willing to work with you.

4. Weapon system is laying in out broad direct sunlight. How many rounds fired and timeframe fired in could account for some elevation deviation due to "preheating" the round in chamber which will cause the MV to vary.

<span style="font-weight: bold"> </span> <span style="font-style: italic"> </span> A good point, the powder used in this gun's case is temperature stable, but not perfect. Chamber temperatures from rapid fire will have more of an effect, sunlight, less so. One of the other issues concerning sun in the early a.m. or on a very cold barrel is the bowing effect caused by the expansion of the molecules "bowing" the barrel away from the position of the sun. It's no little thing, and that is correctable as well. It's a booger though, and YES, the variation is enough to cause a 2 MOA error.

Second discussion example:
1. I am going to go with target is "barely" downhill.

<span style="font-weight: bold"> </span> <span style="font-style: italic"> </span> Barely uphill in the 1st photo, downhill in the second, both angles are within 2 degrees of each other.

2. Potential gun errors??? Has me stumped!!

<span style="font-weight: bold"> </span> <span style="font-style: italic"> </span> I'll leave this one open for thoughts from others. Hint: There are 4 gun/shooter input problem potentials, all of which are managable.

Thoughts??

 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

OK the tailwind will slightly decrease the relative velocity of the projectile which will reduce the drag and bullet drop.

Air speed converges and increases as it strikes the barrier which in this pic is the mountain side.

Isallobaric Wind: Thus far, all has been constant in time, and the winds are steady. Now we consider the case in which the pressure changes with time, and new winds are created. We are familiar with isobars, the lines p = constant, on the weather map. These curves are drawn from the station pressures also shown on the map. The station reports contain another item of pressure information, the "tendency" or change of pressure in 3 hours. The entry 14/ means that the pressure has risen by 1.4 mb in the last 3 hours at the station. We now have a representative value of ?p/?t at each station. These values can be contoured just like the pressure itself, and the lines ?p/?t = constant are called isallobars, lines of constant barometric tendency. Rates of change of pressure contour just like pressure itself, with highs and lows.

If you want isallobars, you usually have to draw them yourself. To do this, put a sheet of tracing paper over the weather map, and at each station put a dot to locate the station, and beside it the tendency at that station, being sure to assign the proper sign, + or -. When you remove the tracing paper, the tendencies will be much easier to interpret than on the cluttered map. If you have enough data, isallobars can then be drawn. We'll show in a minute how to use the resulting map.

We now go back to the equations of motion: du/dt = lv - (1/?)?p/?x and dv/dt = -lu - (1/?)?p/?y. The pressure gradient terms can be expressed in terms of the geostrophic velocities v' and u', so we have du/dt = l(v - v') = lv" and dv/dt = -l(u - u') = -lu". The double-primed quantities v" and u", which must be added to the geostrophic winds v' and u' to get the total winds v and u, are called the isallobaric winds. They are proportional to the acceleration of the air, so are the direct result of forces.

To make the equations soluble, we neglect everything after the first term in the acceleration du/dt = ?u'/?t + ?u"/?t + (terms like u?u/?x, etc). People who have considered this in more detail say this approximation is valid in most ordinary cases, so let's rely on their advice. Then ?u'/?t = -(1/l?)?2p/?y?t = lv", and in the same way, ?v'/?t = - lu". Solving for v" and u", we have the expressions v" = -(1/l2?)?2p/?y?t, and u" = -(1/l2?)?2p/?x?t. In vector notation, v" = -(1/l2?) grad(?p/?t).

The result is then quite simple: the isallobaric wind blows normal to the isallobars from high tendency to low tendency, and the velocity is proportional to the gradient. That is, there is divergence from an isallobaric high and convergence to an isallobaric low. Continuity tells us that in the first case, ther must be a downward flow, and in the other an upward flow. All this depends on rates of change of pressure, not on the pressures themselves.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

What did you just call me?

I am going to have to read that a few more times.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Since you had first said exterior ballistics I discounted chamber heat. Otherwise we could even include the .408's tendency to copper foul if pushed hot.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

as to slight up and slight downhill. Downhill is still in its natural arc and would be more danger space with less possible stringing? Was also thinking the possibility of the wind being colder at the target based on higher elevation or colder air blowing closer to target and a colder wind gust would mean more correction needed?
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Huckleberry</div><div class="ubbcode-body">What did you just call me?

I am going to have to read that a few more times.</div></div>

LMAO well you are from Alabama.....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: The Mechanic</div><div class="ubbcode-body">Since you had first said exterior ballistics I discounted chamber heat. Otherwise we could even include the .408's tendency to copper foul if pushed hot.</div></div>

Fouling is an issue but not a problem in T50s scenario due to having the ability to keep a 1MOA deviation in windage. When they fouled the bullets get slung left to right also lol.



For the life of me I can only think of 2 equipment factors in regards to this scenario:

1. Lateral Jump: Caused by slight lateral and rotational movement of barrel at the instant of firing. Its effect is on "bearing" of projectile as it leaves muzzle.
2. Lateral Throw-Off: Caused by pressure imbalance during the transitorily flight phase when a projectile leaves the barrel. Its effect causes dispersion on the projectile from its intended path.

Shooter errors can be a vast amount of things but you mentioned we are going to assume the rifle and shooter are capable of 1MOA groups so I am still stumped on that one lol.

 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Ok, at risk of sounding like a moron, here are my thoughts on the lower scenario:

The shooter looks to be positioned at a decent angle behind the rifle, which will not allow him to control the recoil pulse as consistently as being straight behind the rifle.

The situation being slightly downhill allows for an angle error, if the shooter is not aware of the elevation difference between himself and the target.

There looks to be some very prominent and sharp terrain features between the target and shooter, sure to cause some issues with reliable wind doping.


And finally, on the first scenario, I am wondering where the wind is forced to change its trajectory from traveling across the ground fairly parallel to the ground and the bullet path, when it strikes the hill and is forced upward, there is sure to be some pressure instability as some of the wind eddies against the slope. Should be some turbulent air at the base of the 45 degree incline, that could potentially back up into the target area...
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Winds from 6 and 12 o'clck will alter the vertical, primarily because they alter time of flight, which allows gravity to work on the projectile for longer and shorter time variables.

If it drops longer, it's probably also going to wind drift further too, and vice-versa.

Greg
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Although it's been roughly addressed.The only thing I can possibly think to ad is this.

Is it morning or evening?
If it's morning the thermals will be rising.Evening-descending.Especially exaggerated at a Mountain slope and increasing or decreasing with the amount of heat up or cool down.

Also combine this phenomenon with the variable speeds of the tail wind and the other factors that everyone has mentioned.

You guys are way too smart.Just a wild guess for me.

Since the shooter and the spotter are having a friendly competition the spotter keeps on nudging the shooters right foot immediately before he shoots.See the shooters right foot apparently touching the spotter in the photo.LOL.But I guess that would cause horizontal issues as well though,huh.LOL

Steve
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

In the first scenario it appears that the wind is funneled by the shape of the mountain range it is impacting, horseshoe. Hence any gust from behind is amplified by the venturi effect and creates a strong varying updraft.

In the second scenario the spotter is not aligned with the shooter, so his calls will be off. Due to the downward slope the shooter is forcing his shoulder into the gun creating hold inconsistencies. Also he cannot obtain a consistent sight picture since his head is rolled over the gun. For old folks like me with progressive bifocals this last one is a killer.

Additionally he is not lined up correctly, i.e. gun axis is not down the leg.

Looking forward to the answer.
Ross
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

I think that the terrain in the first sit. actually is affecting more than just the wind (altough it certainly will affect wind also). Air density could also be effected by such a difference.

Were pressure, temp. altitude etc. readings taken at the target also?
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

Welcome back Trig!
Hmmm..

As mentioned earlier, with the spotty clouds, are some shots taken with full sun entire path of bullet, or are some taken with part of trajectory going through shade? Would this cause local differences in pressure, up/down draft?

Does the rough terrain have sections close to ground with turbulent air flow, and above that area more laminar flow?

I always prefer to spot directly behind shooter, with spotting scope directly above and in same vertical plane. Also as previously mentioned, need to be directly behind rifle, so spine is parallel to axis of bore.

Scott
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

Later...so are you solving using the Conjugate Gradient method?
wink.gif
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

Great post.

+1 regarding the first scenario. How do we compensate for this?

The solution to the second scenario was insightful and accurate. However, what can the shooter do to prevent rolling his head over the rifle?

-Chris
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

Hey shooters,

Been logging on and off for the past few days reading over some things. Some of this stuff is arguable, no doubt. The posts all have good points, some stronger than others. I’ll respond one by one, and hope I don’t miss anyone along the way, my apologies if I do.

Mechanic: 19 June: The updraft part is close, downdraft has no effect that I have seen. More explanation on this in the end statement. Your observation on the danger space is a good point as well. Here is another odd consideration. The angle of incidence at the target affects the shot group size WHEN you have a different point of aim on each shot. Another way of saying this is that a 1 MOA sight adjustment OR hold-off is worth more than the mathematical 1 MOA as the angle of fall at the target increases. It’s the same effect as the COS value when correcting for slant angle in relationship to range. Won’t get into the math of this subject.

The .408 won’t foul excessively at velocities less than 3050 fps. There are other factor however that can increase the fouling. Chamber heat is a good point, but it’s accepted that the MV shot to shot WAS measured on a chronograph to catalog these differences during testing.

Later: 19 June Post: The greatest potential error with the gun/shooter in this photo is that the dang bipod legs are extended too far. Any bipod that is more than about 10” off of the ground, especially when the barrel elevation is pointed uphill (not including the elevation on the scope for the distance / conditions, etc. What happens is that when the bipod is extended too far, once the gun is in recoil, those high legs bow, this also drives the tail of the gun down, causing excessive “gun jump”. The 1 MOA statement does ignore this factor, but it does happen, gotta keep the gun low yes, BUT there is a point of diminishing return where the tail of the gun is TOO low and the bipod legs are too high. The net effect is the tail of the gun goes down, the muzzle jumps. It sucks.
DebosDave: 20 June Post: Touchdown on the shooter interface with the gun, this shooter knows his business, BUT he was forced into the position by the nature of the terrain, the shot, the conditions and to his surprise, the distance that required such a great takeoff angle on the shot. In the photo, the gun looks nose high, but is actually pointed DOWN on about a 2 degree angle, not enough to matter for slant angle range adjustments, BUT enough to make the gun flex a lot under recoil. The suppressor makes this even worse because it slows down the recoil impulse and stretches that out over time, which increases any shooter or gun induced aiming errors.

Greg Langelius: 20 June Post: Greg and others made a reference to the effect of tail winds on the overall flight time of the projectile. It’s not enough to matter, when you convert mph of tailwind into the fps change in the downrange velocity, it’s a very small matter. In cases like that, high BC bullets don’t have great errors at the target. I did an analysis of some shots taken with a 35 mph tailwind and the effect on the supersonic range of the gun/ammo. The change in supersonic range was statistically insignificant, accountable for less than 30 yards of loss in supersonic range.

Rossneder: 21 June Post: One your first scenario explanation, very close, I’ll explain the event below. Second scenario explanation: The variations were in the vertical shape of the group, elevation corrects were NOT given during the groups, only horizontal corrections. Your point is well taken on the horizontal aspects though, good catch. Personally, I hate lying beside a shooter. I prefer to lay directly behind the shooter, if necessary some distance back and talking to the shooter on a low wattage radio net. I might be located up to 20’ away when necessary, but I will have a perfect line of sight down the leg of the shot. This also helps in seeing the “trace” of the shot as you’re above the direct blast wave of the muzzle when the gun goes off.

Gugubica: 21 June Post: Yes, the differences in the air pressure will have an impact on the shape of the group, BUT, it’s not the differences in the barometric air pressure. It’s something else. See below.

Lrs50bmg: 21 June Post: You’re also in the ballpark with the statement on the shape of the ground and wind velocities. Also, addressed below in the closing statement.

Cavemanmoore: Today: The effect of the position on the gun is more horizontal than vertical, BUT it can go vertical for sure if the shooter is bearing down with his melon, which he shouldn’t do. Tension and pressure should be linear with the length of the gun, NOT vertical, where the effect is on the vertical shape of the group. Of course, this is another discussion.

<span style="font-weight: bold"> <span style="font-style: italic"> My explanation: What we have observed over the years. When the wind is travelling downrange, within a 20 degree angle to either side of the gun target line, AND when there is rising ground at the target, not just a little rise, but an entire hill mass, such as the hills seen in the photos. The effect is that the running wind COMPRESSES on the surface of the hill and then runs over the top. This is not a change in the barometric pressure of course. The net effect is that long high BC VLD type bullets, 408, 375/408, VLD .50 calibers like the AMAX, etc. cannot settle easily into that pressurized zone. It’s like an airplane trying to land on a hot runway, you have to almost FORCE the airplane to land. Granted, if time is part of the formula, then the effect would be small. In the first photo, the rising ground is on about a 10 degree angle. When holding exactly the same elevation hold, in winds of 15 – 20 mph (for example), the pressure zone increases and decreases a few times while the bullet is in flight, then POOF, vertical stringing that’ll drive a saint to drink. Is it predictable, Yes. Is it manageable, probably not unless you have the option to NOT shoot when these conditions exist. It is such a problem that the error potential is greater than a windage error at these ranges. As with anything in the extreme range game, the last 25% of the supersonic range of the shot is when these effects must be managed to a high degree and NOTHING can be ignored.
What do you guys think about management options?

Thanks for contributing, good answers.

Trigger</span> </span>
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

Management options.
1. specialized ammo? since range is still pretty low for a .408, maybe a less BC bullet that may be less prone to this condition?
2. if it is not a target of opportunity maybe careful MET monitoring to get best time of day or night for less effects on projectile?
3. better location for shot that will give a better position for shot?
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

On #1: Your point is valid, cept at a lower BC, you've got to be closer to the booger eaters, hence why mess with a special cartridge. The Sierra MK version of the 350 gr. 375 CheyTac would do that scenario well. It's a standard, nothing special Sierra MK that looks funny to me. It's not performance enhanced as far as I can tell. Have to radar shoot it to be sure. It's a cheaper alternative though that gives away some supersonic range to the solid.

#2: Agreed. Night time is always the best time for stable air conditions and for a relaxed booger eater to be more open to the idea of falling over to assume ambient air temperature.

#3: Also agreed. A higher angle of attack will always give you an advantage in the field of exterior ballistics. Less elevation due to gravity effect, and as far as this scenario goes, you have a better penetration of that compressed air layer at the surface. All things add up in the shooter's favor. Always remember though, and lots forget, as far as windage goes, you have to shoot winds as the real distance is calculated, NOT the slant range. 1000 yards on a 45 degree angle is 770 yards, give or take, for elevation, but you still need 1000 yards worth of windage.

As far as position, i'd rather nose a gun down, vs. nosing a gun up. BOTH tail high and tail low positions can effect the sheaf of shots on the ground, but the gun tail low position is more painful (I think), unless of course "she" is your girlfriend or wife, and the "she" as in rifle
smile.gif
Sorry, couldn't resist!!

Trigger
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

I agree the only realistic management option is projectile choice. I'm not sure that controlling the conditions themselves is a valuable solution. Our common goal is to become the best marksmen we can. For me, this means hitting my target under as many different variables as I and my equipment are able. The conditions are not something that we should place excess emphasis on controlling. We could move much closer, for example. But that would take all the fun out of it. At the far end of the range the only thing we can control is the predetermined choice of projectile. Do unlike bullets react to Trigger's condition differently? Absolutely. But would we want a bullet with a higher or lower BC?
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

Throw in my two cents here. Vertical stringing is usually a component of velocity once wind is ruled out. Are we sure those shots were exactly the same speed. How would we know the ES?

Also, Heat on the barrel. Not chamber heat or cooking the round but the barrel steel moving from heat. Doesn't have to move much to make a big difference at that range and if you have that Can on there it gets hot!
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

If you have the time under military conditions (not a go light) you would want to shoot between gusts of wind etc. and if you have time you would want to make as many variables as small as possible. Enemy not facing you, taking a dump, night time, eating, highest value target, etc. so taking into consideration MET's I think is valuable, predictable, and can be compensated for. Practice under all conditions is important but not as important as killing the enemy as easily as possible while still being able to get away IMHO. As for .50 shooting the choice of projectiles is huge.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: cavemanmoore</div><div class="ubbcode-body">I agree the only realistic management option is projectile choice. I'm not sure that controlling the conditions themselves is a valuable solution. Our common goal is to become the best marksmen we can. For me, this means hitting my target under as many different variables as I and my equipment are able. The conditions are not something that we should place excess emphasis on controlling. We could move much closer, for example. But that would take all the fun out of it. At the far end of the range the only thing we can control is the predetermined choice of projectile. Do unlike bullets react to Trigger's condition differently? Absolutely. But would we want a bullet with a higher or lower BC? </div></div>
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

When I start seeing mathematical symbols, my eyes tend to glaze over...

I usually have to flog myself to read through and understand these things, not because I'm in need of computing specific values, but because the data outputs over a range of inputs helps me gain intutition into how variables affect outcomes.

My experiences regarding shooting over rolling terrain are that it almost invariably presents vertical voodoo. Not a very scientific term, I wager, but descriptive of the issue to a 'T'.

My own surmise is that the varying heights of the trajectory subjects the projectile to stronger influences as vertical distances increase, and that these higher magnitude influences are subject to greater overall variations, and nearly always absent of any visible cues. These variations correspond to air movements in all three axes, and often in multiple different directions at any given instant. Couple this with a strong likelihood that no two sets of variables are the same, and the vertical voodo emerges triumphant.

I think it may be a futile task to try and either explain or predict these outcomes. For my purposes, it suffices to expect that dispersion will be proportionally relative to terrain convolutions, and leave it at that. Basic rule of thumb: when one is in the hills and wants to hit it precisiely, get closer.

BTW, this insight is not my own pipe dream, but the essence of several articles in <span style="font-style: italic">Precision Shooting</span> about shooting in the SW ravines, from a decade or two back.

Greg
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Mechanic: Your point is a good one. At extreme ranges in a military environment, or whatever environment you're shooting at something that is living, flight time is everything (to me).

I evaluate everything from a "how long does the bullet take to get there, and how long is that (target) going to stand in one place". I evaluate something I call Motion Exposure. If I can do 1 MOA for example, how much of the target's 1 MOA mass going to remain in that 1 MOA circle. Does the target shift back and forth on the feet? Does the target wander about? Things like that.

Now,granted if the target is a piece of steel, flight time is less of an issue. Part of target analysis (military point of view) and not combat sniping but specific point target engagements, require target analysis. Some air breathing carbon based units are 600 yard targets under any condition, they are constantly on the move. With that, some are also 2000 yard targets.

The 375 Cheytac can go 2500 yards in 4 seconds under standard conditions. That still is a long time, yes, the gun CAN nail a 20" wide target at that range, it's been done, but will the non-steel target be there when the bullet arrives in that 1 MOA arc?

Greg: Fellow combat engineer (late 70s anyway)... Good observation on the max ordinate effect. Even the great 375 Cheytac, at 2500 yards over flat ground is 75 ft in the air, where I train at the max ord is 63 ft. The winds at that height ARE stronger that at the ground, where we tend to use mirage as an indicator. When using the Cheytac ABC computer with 3 windbands, the midpoint winds ARE value adjusted taking into account the max ord of the shot and adjusts the shooters mid range windage input. Of course, it can't adjust for the height of the shot over dipping terrain, etc.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I think it may be a futile task to try and either explain or predict these outcomes. For my purposes, it suffices to expect that dispersion will be proportionally relative to terrain convolutions, and leave it at that. Basic rule of thumb: when one is in the hills and wants to hit it precisiely, get closer. </div></div>

On your statement here, I agree 110%. Sometimes you just gotta scratch your head, kind of like when the Army selected the Knight's M110 for their observer's gun. A belt fed, optically sight fitted and suppressed belt fed machine gun, a.k.a. SFOD's H&K-21 from the 80s and 90s is the best backup gun. Or 2nd best, the M-25 rifle as specified by Tom Kapp of 10th Special Forces Group in the 80s. Great rifle, that is nowhere made today.

My .02 cents...

Trigger
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Triggerfifty</div><div class="ubbcode-body">On #1: Your point is valid, cept at a lower BC, you've got to be closer to the booger eaters, hence why mess with a special cartridge. The Sierra MK version of the 350 gr. 375 CheyTac would do that scenario well. It's a standard, nothing special Sierra MK that looks funny to me. It's not performance enhanced as far as I can tell. Have to radar shoot it to be sure. It's a cheaper alternative though that gives away some supersonic range to the solid.


Trigger </div></div>

The 260 or 300 grain accubond would be a good choice. I have shot the 260 and am gettin less then 15 moa to get to 1k.
 
Re: NEW TOPIC: Increased Air Pressure on the Fron

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: StrategicEdge</div><div class="ubbcode-body">Throw in my two cents here. Vertical stringing is usually a component of velocity once wind is ruled out. Are we sure those shots were exactly the same speed. How would we know the ES?

</div></div>

That is not the case all the time. I have shot groups of 10 and had a Standard Deviation of less then 4 and at 1k the group was 18" high. Its a consistent 6-7" 1k gun.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Triggerfifty wrote
>>The effect is that the running wind COMPRESSES on the surface of the hill and then runs over the top. This is not a change in the barometric pressure of course. The net effect is that long high BC VLD type bullets, 408, 375/408, VLD .50 calibers like the AMAX, etc. cannot settle easily into that pressurized zone. It’s like an airplane trying to land on a hot runway, you have to almost FORCE the airplane to land.


Interesting premise, but the physics don't support it. Ground effect is the result of a reduction in drag, due to the reduction of wingtip vortices. This occurs in a very small area, approximatly 1 to 1.5 wingspans off the ground.

The lift component of a ballistic projectile is vanishingly small, and, as it's 'wingspan' would effectivly be the diameter of the bullet, to the extent the effect might occur at all it would be within 2 calibers of the ground. As the bullet at extreme ranges is point down, nearly tracking the ballistic arc, it will be penetrating the air below it just as well as the air above.

Absent shooter errors that have already been discussed, what would be the issue? The most likely candidate for such vertical stringing in the stated conditions is mirage. The target image is being shifted up and down by the head/tail wind flow along the angled surfaces. The shooter is simply not pointing the gun in the same place twice, he just does not know it.

A pure head/tail wind has virtually no effect on the ballistic trajectory. To the extent that there is a vertical component, such as a severe slope in front of the shooter, it's value is the same as a crosswind correction, just in the vertical plane. Bullets in a crosswind do not get 'pushed sideways', the bullet turns to point into the relative wind vector. Excepting very steep terrain and strong winds, such up/down slope wind effects are not typically very deep, perhaps a few hundred yards. They are very difficult to evaluate though, as there is little to visually indicate the speed and actual depth of the effect.

I'll also have to quibble about the suppressor having an effect on the issue via 'extending the recoil impulse'. While the suppressor may have some small effect prior to the bullet exiting the muzzle due to the air forced ahead of the bullet, once again we are talking about an insanely small effect, measured in milliseconds, the time to bullet exit. Even if you used the full bore volume of gas giving the full possible effect, it's nearly too small measure. The recoil reduction of the suppressor, or the muzzle brake itself, is not actually in play until the bullet has exited and the propellent gasses work against the device surfaces. At that point, nothing the shooter does can have any effect on the bullet, it's left the rifle. Positional errors and lack of followthrough are an issue because the shooter ends up moving BEFORE the bullet has exited, that will be a problem. PIP (pre-ignition push) is a more likely suspect, expecially as the recoil forces go up.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Interesting thread...
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Cory,

Quibble all you want, you're wrong all your observations. How much mountain shooting time do you do? At ranges that are within the last 15-25% of supersonic? Suppressor comment, sorry, but measured with highspeed video UNDER controlled conditions, those recoil impulses DO HAVE AN EFFECT on trajectory, as does piss poor muzzle break design (barrett arrowhead type for example), Phil Seeberger documented this with HP White and Aberdeen PG in the 80s, it's old history.

I have seen these events taking place since doing this game in the mid 80s with the early 50s, 14.5s etc. They all occur.

Mirage on the long range scenario, sure if I said any mirage was present, which it was not in the scenarios observed in about 14 foreign countries on varying terrain, not just a flat desert in the US. Theories are one thing, observations based on real events rise above theories in most cases. Theories usually don't fly past evidence based scenarios or real observations. Funny though, in those cases where the winds do let off (which usually brings mirage back into the picture, winds eliminate the mirage), the event stops occuring, vertical footprint settles down, the group becomes more predictable. THE only thing that changes is the wind downrange stopping.

Yes, I have had guys downrange with radios to measure this event as well to confirm or deny the wind conditions there. This isn't some 1 or 2 time event. It's been occuring for many moons
smile.gif


Later,
Trigger
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Concerning this observation...

"A pure head/tail wind has virtually no effect on the ballistic trajectory. To the extent that there is a vertical component, such as a severe slope in front of the shooter, it's value is the same as a crosswind correction, just in the vertical plane. Bullets in a crosswind do not get 'pushed sideways', the bullet turns to point into the relative wind vector. Excepting very steep terrain and strong winds, such up/down slope wind effects are not typically very deep, perhaps a few hundred yards. They are very difficult to evaluate though, as there is little to visually indicate the speed and actual depth of the effect."

1 mph is about 1.5 fps, so a 20 mph tailwind is a net effect of 30 fps effect on the MV. Depending on the bullet, this is about .5 to 1.5 MOA at the target, so not too much concerning the discussion, but enough to be a miss long or short, so it MUST be considered, especially in that dreaded NSST (near sub-sonic transition) range.

Bullets DO NOT turn nose into the wind. What you're suggesting is that the size of the projectile in the rear is pushed because it's larger than the nose? If that's the case, then the bullet is most certainly being pushed sideways. As measured on radar, a bullet MAY nose into the wind, but only if it's correctly stabilized (balanced flight), and the nose is nosing over as it passes over maximum ordinate, then yes, then phenomonon has been observed recorded and measured, without a correct nose over attitude, the projectile will not "turn into the wind". I'd ask where did you discover your observation about bullets not being pushed by the wind. It has long been accepted and measured that bullets do pick up a crosswind velocity immediately as the leave the gun, it's less at the near ranges, but greater downrange and past MAX ORD.

On your last statement about difficult to measure the effect. Try using smoke, it provides the visual effect.

Trigger
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Cory,

Just re-read my post, I apologize for being so blunt, I meant no disrespect to you. Sorry about that. The past year has made me a bit cranky!!
smile.gif


Trig
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Maybe I missed this as an explanation, but wind flows up and over a mountain range creating a lift component. The bullet is essentially nose down and therefore more surface area is exposed to a wind that is running parallel to the ground. The wind is not pushing against the tail of the bullet, it is pushing on the side of the bullet.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

TF, I did not deny your observations, only the reasons behind the occurence.

The suppressor can have no effect on the TRAJECTORY, beyond whatever freebore boost may add to the velocity. Trajectory is mainly determined by the atmosphere, velocity, BC and departure angle. Certainly it will change the POI, but not the trajectory itself. Accuracy and precision may be improved or degraded by poor design of a suppressor or muzzle break, BUT, these effects take place AFTER the bullet has left the bore. Perhaps the cleaner exit column sets up a bullet with less precession, leading to improved accuracy. Perhaps the base is less (or more) disturbed by the baffles stripping away the exiting gasses, thereby altering the incipient yaw for better or worse.

That is not a result of a 'lengthened recoil impulse'. That suppressors do mitigage recoil over a longer period I did not deny, only that that is not cause and effect. Does the suppressor have an effect on the shooter and the POI, yes! It's just not related to the recoil impulse prior to the bullet exiting the barrel.

I did not say the bullet turns into the wind, I said it turns into the RELATIVE WIND. Whatever crosswind velocity component it picks up is quite small, but the bullet has moved it's tip to streamline with the air mass. Thus, it turns UPWIND, therby following the crosswind vector formed by the velocity, direction of departure and wind direction and velocity.

http://www.exteriorballistics.com/ebexplained/4th/53.cfm

That is why a near wind has more effect on the deflection than a far wind. Once the angle off has been established, a counter force would be required to return the bullet to the original departure vector. You already know that.

http://www.exteriorballistics.com/ebexplained/4th/532.cfm
or see Bullet's Flight by Mann, or Understanding Firearms Ballistics by Rinker.

The tailwind DOES NOT increase the velocity, it effectivly reduces the drag. In point of fact, relative to the air (which is all that matters), the bullets velocity is DECREASED.

For a detailed explanation

http://www.exteriorballistics.com/ebexplained/4th/531.cfm

The effect of a 20 mph tailwind on a 1500 yard shot is about .1 mil less in elevation, NOT 1.5 MOA. Try running the numbers on any ballistic calculator you may have, they all come out about the same. QuickTarget, JBM, FFS, Exbal, Atrag, and Infinity all provide about the same answer

When I said it's difficult to evaluate the effect, I was referring to the absence of visual cues. Certainly if you could pop smoke at a number of points along the shot, that would be nice, but seldom practial, as you well know.

Once again, I'm not denying the observations, nor the issues. If I'm somehow wrong on the physics, I'd like to be shown HOW I'm wrong. All the sources I've ever seen support my statements, some of which I've posted above. If you got another reference source that says otherwise, I'm always open to another explanation. Don't just tell me I'm wrong, you've seen it happen. Show me the math, video or other science that explains WHY I'm wrong.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

And no apology required, I don't read insults into a persons reasoned response. We usually agree on observered reality. In this case, we are debating the reasons for the occurance. It's certainly possible we'll end up agreeing to disagree. If the solutions to the observered difficulty work, in the end that's the big issue.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Huck, the side of the bullet really does not provide much surface for a 'push' to occur. Nor would it be correct to describe it as 'lift', as in the wing of an aircraft.

Check the links above, I've got to cut myself short here and go downrange and shoot. Rats!
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Cory,
I agree not much surface, but a side wind reacts on this same surface and moves the bullet several feet.

If the bullet is noise down at a 20 degree angle and the wind is moving up a slope from a lower elevation, then some bullet movement would have to occur. The bullet is not lifting so much as being pushed further forward in the direction of the wind.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Just a quick note on these ballistic calculators, they ALL use a singular BC value, based on some factory or a near downrange remaining velocity calculation.

With the exception of the CheyTac ABC / CABC, none of these calculators are remotedly accurate in the 25% to supersonic transition range, that is the key figure. The singular calculators have as much as a 5-8 MOA error in the near subsonic range and are NOT reliable for that near transonic calculation. No scientific authority, YPG, APG, etc. use any program but their own downrange radar data because they know the BC values change radically, which you know.

Trig
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Spotter says wind is going 6-12, but holding meter @ 9:00.
"You can see the aiming target 8' X 12' white square down range"
"Range to target is 1500 - 1800 yards."
Pic two shows spotter looking at target also. Not looking for what the wind may be doing in the big gap in between.
Assume nothing, is what I believe you originaly said.
".408 chy. of course." You could go smaller with both and get more trigger time.
Eliminate a couple more variables by using a 12" circle @ a mile.
If you need to hang a target that far, there is a 4x8 size holder for you.
Black disk left, White disk right.
Able to get dope for all at first station.
550,950,1150,875.(optional are 1720, and mystery)
Three per target max, and eight min. (10,5,2)
48 rounds for the course.

By the way, wind from six pushes it down, and from 12 gives it lift here.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Trig, true on the BC values changing as the velocity decays. A number of the systems use a multi BC table to account for this, some use a deceleration constant to alter the curve. QuickTarget will use Doppler radar derived Cd's for it's predictions. Mr. Paver is sending me a copy of the Military ABC software, but when I last looked at it a year ago it would come up with the same tailwind/headwind solution as the others I mentioned.

The bullet does not get the extra 30 FPS in velocity from the tailwind. Adding 30 FPS can certainly get you a .5 to 1.5 MOA change in POI, but that's not what happens.

For what it's worth, I've field tested Field Firing Solutions out to 2000 meters with a 250gr Lapua Scenar in the LM case, which is a fair but below subsonic at that range and the calculated elevation is within .1 mil.

We have matched the predicted TOF, drop, drift and velocities to actual Doppler recorded data and the numbers are within a couple milliseconds for TOF, an inch or two for drop/drift and a couple meters per second for velocity.

Huck, once again, note that the bullet is not being pushed as much as it's being turned to streamline itself as a result of drag. For a vertical wind, that is essentialy the same as a side wind, just tipping the nose up or down.

I sould also take this chance to explain my comment on mirage. Mirage is simply a variation of air density, normally caused by heating of air at the surface. The density variation has a lens effect, distorting the image as viewd throught he scope. A vertical air movement can cuase the same effects, moving warmer or cooler air and creating a varied density, refracting light and cause an image to appear to shift either up/down. In such a case, the characteristic wavy lines of heat shimmer will not be present, leading some to believe the effect is not present. Fixing a scope to a solid object and viewing the apparent shift of a fixed target allows one to see it happen. Image shift is a potential problem unless you are in a vacuum, where such refraction will not take place.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

TCA4570, bullets don't have a measureable component of lift. A direct headwind will cause the shot to strike low, a direct tailwind will cause the strike to be high. Both of these effects are very small. Of course, winds are rarely directly anything, nor are they precisely measured. This causes all sorts of problems with "Well, I was there and saw this happen." I assure you, much smarter people than I, with really cool precision equipment, have actually 'done the math' as it were.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Cory,
which ones are using multiple BC's? I know patagonia does not and I don't think Horus does, and I don't think FFS does. I know most use a deceleration constant but that is only a "rounding" of numbers to get a little high close, a little low at long range. I know ABC uses I think 2 or 3 separate BC's at specified distances.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

does that include spin drift or Magnus effect? And I understand this has no inclusion on anything under a min of 1,000. As to bullets turning into the wind could that be just "yaw of repose"? just happens to be in the direction of wind? Maybe just coincidental during testing?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: CoryT</div><div class="ubbcode-body">TCA4570, bullets don't have a measureable component of lift. A direct headwind will cause the shot to strike low, a direct tailwind will cause the strike to be high. Both of these effects are very small. Of course, winds are rarely directly anything, nor are they precisely measured. This causes all sorts of problems with "Well, I was there and saw this happen." I assure you, much smarter people than I, with really cool precision equipment, have actually 'done the math' as it were. </div></div>
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">which ones are using multiple BC's?</div></div>

Nightforce/Exbal and Sierra use multiple BCs in velocity ranges.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I know most use a deceleration constant but that is only a "rounding" of numbers to get a little high close, a little low at long range.</div></div>

The proper use of a deceleration constant is to ensure that the program predictions match actual shooting data at long ranges, where accuracy is most important.
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Mechanic and other shooters,

The Cheytac ABC uses a different BC value every 100 yards or meters. As the range is input, the program calculates and extracts the exact BC value between listed values (interpolation), then corrects that BC based on the meteorological inputs the operator puts in. Here is a string of BC values for a particular bullet, note the variations.

100 0.656
200 0.675
300 0.683
400 0.688
500 0.691
600 0.693
700 0.694
800 0.695
900 0.696
1000 0.696
1100 0.695
1100 0.693
1200 0.689
1300 0.687
1400 0.687
1500 0.698
1600 0.712
1700 0.725
1800 0.736
1900 0.745
2000 0.751
2100 0.754
2200 0.754
2300 0.752
2400 0.750
2500 0.746
2600 0.740
2700 0.734
2800 0.727
 
Re: NEW TOPIC: Increased Air Pressure on the Front ..

Not to bug ya' Lindy but would you want to change the DK at regular intervals based on decay? Is the DK truly only good "read perfect" at one specific distance? One more question. Is that the new Sierra? I have 5.0 I think and I don't remember there being that ability?

Dean,
does ABC adjust for MV changes? Like hotter load "temp" or handloading? Is there that input?
WOW hits peak BC at 2000?
 
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