Unexplained Velocity Change

[Bigwheels]

I have been trying to figure out a change in velocity. I have been out shooting @ 1250-1650 yd 300 miles, & a few thousand feet elevation apart. Both ffs, & shooter app are tuned to my load @ "home range" but it happened that on the last two outings I had very similar temps at very different elevations, & both my ballistic calculators needed to be "trued" to match up with actual impacts @ the higher elevation. I use a kestral to enter station pressure, temp, & humidity for calculating come ups. The load is the same lot loaded on the same scale, from the same lot of bullets, etc. The only thing I can think of to account for this is a mv change. At higher elevation I was hitting higher than predicted, & shot lower again at lower elevation again. Could lower station pressure cause an increase in velocity?

 

[eracer]

Re: Unexplained Velocity Change

Lower station pressure = air is less dense = less drag on bullet. Quite noticeable over the yardages you shoot, methinks.

Not sure why ballistic app isn't compensating.

Bad data?

 

[distantfoe]

Re: Unexplained Velocity Change

A guess, burn rate increased with elevation jump = increase in velocity??

 

[MALLARD]

Re: Unexplained Velocity Change

are you using the g7 or g1

 

[Bigwheels]

Re: Unexplained Velocity Change

I am using both really as shooter app can use the g7 but ffs can only use the g1. This is my fundamental question. Why would both calculaters not track properly with an elevation change? I use station pressure with my kestral for all my entries.
Another question is just how do you get the most accurate temp readings? I find that if I hold the kestral sideways to the wind I get a higher reading than if I hold it facing the wind, as when I'm taking wind readings. I have been using the latter which gives the colder reading.
I may try to use the density altitude reading that the kestral offers, & try that. However after entering a d/a into shooter app with different results, but not the right one. I'm still confused.

 

[jrob300]

Re: Unexplained Velocity Change

Bigwheels,

Stupid question... any chance your ammo was sitting in the sun? I've gotten unexpected elevation and was really scratching my head until I realized the sun had moved and my ammo box was baking.

Other than that I'm baffled.

John

 

[Bigwheels]

Re: Unexplained Velocity Change

I believe there are no stupid questions, but to answer. No I was in the shade under some trees, & it was mostly clowdy. Temp was 58* on both days. Humidity was a little lower at high elevation but other than that the only thing that changed was the location, & station pressure.
I am using the Hornady 225's, & the posted BC MUST be verry low, as when I tried the .670 G1 BC with the MV given by 2 chrony's I was hitting a LOT high, so I am using a G1 of .710 in FFS, & a G7 of .350 in Shooter with a MV ranging from 2830-2860 depending on where I am.(High, or low elevation) MY 2 chrony's are cheap, but they both gave an average of 2838fps.
The weirdest thing is the DA card I built after first working up the load is the closest thing I have right now. But it is also off (shoots high) at high elevations.

 

[SONIC SAAMI]

Re: Unexplained Velocity Change

Have you looked at this?
http://www.snipershide.com/forum/ubbthre...317#Post3330317

 

[distantfoe]

Re: Unexplained Velocity Change

Have you confirmed your come ups on your home range at ranges 1250-1650yrds? If you have then velocity must have changed.

 

[Bigwheels]

Re: Unexplained Velocity Change

I have confirmed @ 1250 on my home range, but it only goes to 1250 so I can't check the longer shots.
If the mv did change, I can't think of anything other than the station pressure to cause it. If that's the case then now to compensate?

 

[PeterPHWS]

Re: Unexplained Velocity Change

The only way to work these things out is "elimination" of variables ... you don't say how much extra velocity would be needed to account for the elevation change ... nor do you say what terrain you were shooting over ... nor the time of day or how warm it was ... or what power you were shooting on

There are a whole host of "gremlins" that can give elevation issues which are hard to fathom ...

I have had velocity changes from the same batched lot of powder simply by opening a new tub ... powder can degrade and lose velocity simply based on shelf life ... the longer you have it the slower it can drop ... even from the same batch ... then when you open a new tub and break the seal the new tubb is often a bit more pokey ... I chrono therefore for every tub if doing a comp and try to get all my loads from that tub for the comp.

Then there is the terrain ... shoot at Bisley on Stickledown and the firing point is all "grass" ... at 1200 yards using a kestrel you will get a totally different set of figures at the firing point compared to that part of the range which has peat and heather over which you are shooting ... start at 8.30 am and you can shoot a possible ... by 9.30 to 10.00 am once the sun has been up a while the overnight dew and greater water content of the peat cause evaporation which can play havoc with elevation as the air pressure/density there is a lot different to the firing point.

Then there are other aspects on terrain ... again using stickledown as an example ... the firing points are on the side of a hill slopingright to left ... at the bottom of the hill if the wind is left to right you are shaded by the trees ... it is always less wind but always more elevation ... going up the hill it is more wind and less elevation ... the wind not only deflects the bullet but lifts it ... so terrain can account a great deal for elevation changes ...

Then there is atmospheric aboration based on air thinness or mirage ... both play a part ... they basically alter the way light is refelcted from the target ... this can cause changes in what you think is your point of aim and what really is your point of aim ... go to Ottawa on the Connaught Range in summer in Canada and set up a mil dot reticle spotting scope on the Bull at only 1000 yards in the early morning before the sun gets up and is really hot ... then watch and see how the bull moves around the scope as the heat builds up throughout the day ... even without a boil or low wind/mirage impact ...

Then there is the issue of shooting on 15x up to 20x where the mirage is often not appreciated ... check it on 30x and above and you can see clearly when there is a "boil" ... shoot in a boil and you can have some serious elevation issues ...

All of these issues will not be pointed out by your Kestrel and ballistic calculators ... all can give elevation variables ...

Reading the temp data given later 58f is 14.5c ( I think in celsius ) so it is'nt too hot ... and is a temp band where optically you should'nt have issues with POA shift due to heat ... so my best guess here is that there will be noticeable terrain differences ...

From your home range to where you were shooting you need to think through the terrain variables from the direction the wind is coming from ... where you got higher elevation than expected I would anticipate that the wind could be coming from a direction where it might roll up a hill ( you say you were at a higher altitude ) and if you were just on the edge of a tree line ... wind patterns circle and roll up over trees ... these geographic issues cause not just lateral movement on the bullet but also "lift" ...

At ELR it takes a lot of time on a given location to figure out what is happenning with the wind ... many of the top coaches will pop smoke flares if they can to fathom what the variables are on a new range ... particularly when the wind is coming from an unusual direction ... just so that when it comes to a comp they can factor in these sort of differences ...

I would'nt put the change solely down to your ballistic software and kestrel info ... so much at ELR goes beyond what they can deliver ...

 

[Bigwheels]

Re: Unexplained Velocity Change

Thanks. I'll try to go over some of your observations as best I can without access to my databook. My home range is along a hillside in a clear cut with a gully about 3/4 of the way to 1250. I see updraft there a lot, & have learned to compensate for it. As i live in western Wa. I usually have high humidity, & really strange winds. However I have on several occasions been able to shoot there in as close to no wind as you will find in nature so I have good dope, & trued my software at that time. The area I was last at was on the dry side of the state but there were showers around that day so the humidity was similar. Also to set up the tgt @ 1650 I had to walk it all the way over there, & witnessed the winds while hunting the same place for several hours earlier. If anything there should have been a slight down draft present. The area is much flatter than my home range with a gully about 1/4 of the way to 1650. I have shot there on several occasions before, as this is where I hunt. I will consider the age of powder as possibly being the culprit. I hadn't thought of that. However I have had similar issues going from my home range to 5000' also. I just haven't been able to narrow down a source yet.
Keep the ideas coming guys. Tonight I am re-testing my scope calibration, & will re-chrony asap as well as check my zero.

 

[distantfoe]

Re: Unexplained Velocity Change

Definately would be worth running across the chrono.

 

[Tribe]

Re: Unexplained Velocity Change

Bigs,
Will the dope match up (within reason) at the multiple locations/conditions, by tweeking the BC?

 

[Bigwheels]

Re: Unexplained Velocity Change

I just finished up with the scope calibration check. This time I clamped the rifle in a vice attached to a 3000# concrete block, & adjusted it to be rock solid on the zero line. My scope is off by 3 clicks @ 50 moa. I admit I wasn't as anal when I checked it before. After making that adjustment in the software I'm a lot closer. So I went back through my data book, & tweaked the mv, & bc to match up with observations, & I think I have this licked. I'll check my zero,& chrony as soon as I get the time. Thanks guys. I hope I have this figured out now, & I'll check back in after i finish with the rest.

 

[Tribe]

Re: Unexplained Velocity Change

Wow.... I bet that concrete block, was a bitch to drag out to the range...

 

[PeterPHWS]

Re: Unexplained Velocity Change

Scope variables! Well that has caught me out too ... and I agree it is hard to fully check out a scope ... you can do the basics from an anchored steady position as you have done ( and I do to ) just to check the tracking is consistant and also to see if the reticle stops moving before the turrets stop clicking at the extreme's of the movement. These show up doing that. There are harder things to find out though and FWIW here IMO you need to be able to shoot to learn them.

I put my scopes on my lower calibre rifles which are real "tac drivers" like my 6ppc or 6.5 x 47 and test the scope in a tunnel range to eliminate wind.

Do this and you determine the true click value of the scope ... none are ever a true .25 MOA ... then bracket the movement in 10 MOA steps ... usually you find the top and bottom 10 MOA on a turret is where you encounter the greatest shift in the true click value and can work out what this is ... then if the scope is SFP alter the power band and record the POI shift ... now that test is a real eye opener !

 

[Bigwheels]

Re: Unexplained Velocity Change

I wish I had access to an underground range. I also don't have any small caliber rifles. But I have a .192 capable load in this one. This time in the vice I had a helper to turn the turrets while I watched, & I had measured lines every 5 moa. I didn't see any variation in the movement. I noticed about 1 click error about 15 moa, it tracked to 50 moa with 3 clicks to far at that point. So the error was 1.01522. I had to bring the rifle to my old work at a boat yard to do this test. Luckily I left them on good terms. I wish I had though of this a long time ago. Oh well now I know.

 

[PeterPHWS]

Re: Unexplained Velocity Change

The "tunnel" ranges are not really "underground tunnels" ... these are made by using pre-formed concrete drainage pipes for highway construction ... basically joined together for 100 yards with a hut at either end. One for the back stop and to put up targets ... the other for a shooting position ... the diameter of the pipe is enough to walk down hunched over a bit ( for me ) so is probably about 6 ft. Cover the pipes with soil once joined and cemented together and you're good to go.

We use extractor fans and CCTV to make sure the air is clear and so you can see the targets well if not using scopes and want to check your group.

The budget for this type of facility is not "huge" and easily repays itself quickly ... there are two of these in the UK that I know of and both are seriously popular for the obvious reasons ...

If you have "land" and a good local shooting community ... price up doing a range like this. Our club wishes it had done two sets of pipes at the start ... the extra cost would have been well worth it in hind sight ... if I had the money this would be my "retirement project" ... putting a twin tunnel together on land you could go out to 2400m on ...

 

[mdavis78102]

Re: Unexplained Velocity Change

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: PeterWilson</div><div class="ubbcode-body">The "tunnel" ranges are not really "underground tunnels" ... these are made by using pre-formed concrete drainage pipes for highway construction ... basically joined together for 100 yards with a hut at either end. One for the back stop and to put up targets ... the other for a shooting position ... the diameter of the pipe is enough to walk down hunched over a bit ( for me ) so is probably about 6 ft. Cover the pipes with soil once joined and cemented together and you're good to go.

We use extractor fans and CCTV to make sure the air is clear and so you can see the targets well if not using scopes and want to check your group.

The budget for this type of facility is not "huge" and easily repays itself quickly ... there are two of these in the UK that I know of and both are seriously popular for the obvious reasons ...

If you have "land" and a good local shooting community ... price up doing a range like this. Our club wishes it had done two sets of pipes at the start ... the extra cost would have been well worth it in hind sight ... if I had the money this would be my "retirement project" ... putting a twin tunnel together on land you could go out to 2400m on ... </div></div>

Can you imagine how tall the tunnel would have to be to get a bullet to 2400 meters?

 

[Tomcat088]

Re: Unexplained Velocity Change

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mdavis78102</div><div class="ubbcode-body">Can you imagine how tall the tunnel would have to be to get a bullet to 2400 meters? </div></div>

That's exactly what I was thinking. Ain't gonna happen, lol.

 

[PeterPHWS]

Re: Unexplained Velocity Change

LOL ...

It's badly worded I grant you ... what I meant was have a 100 yard set of tunnels for guys to calibrate their scopes ... chrono their rounds ... check their E.S.'s and then go out and be able to shoot at ELR ...

A "one stop" range facility so to speak where you could do all you needed to ...

 

[Arbiter]

Re: Unexplained Velocity Change

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tomcat088</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mdavis78102</div><div class="ubbcode-body">Can you imagine how tall the tunnel would have to be to get a bullet to 2400 meters? </div></div>

That's exactly what I was thinking. Ain't gonna happen, lol.</div></div>

Just build the tunnels up and down both sides of a hill, and aim your scope into the bottom of the tunnel a few hundred yards out. You may not be able to see your target, but you'd still be able to hit it.

 

[VKC]

Re: Unexplained Velocity Change

Since it hasn't been mentioned...how about cant and angle?