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