Gene,
I expected someone to counter with the air mass and atmospheric pressure arguments.
You are completely correct that air has to be pushed out the barrel along with the bullet. But the effect of changes are so small that the effect is insignificant.
A back-of-the-envelope calculation (from a thread sometime last summer): the difference in mass of the air inside a 0.308” barrel 22’ long with a change in temperature from 20C to 30C is about 0.016 grains. Thus the changes caused by the change in air mass should be about the same as that if the bullet were 0.016 grains lighter. That is 0.014% difference for the 118 grain bullets that I shoot. Muzzle velocity, for instance, would go from 3050 fps to 3050.4. Not enough to affect tune.
Lets check the possible effect of atmospheric pressure, too. 2 psi would be a very large local pressure change. A 2 psi drop would change the pressure difference driving the bullet out the barrel (50,000 psi chamber pressure minus atmospheric pressure) by only 0.004%, and increase muzzle velocity from 3050 to 3050.1. Again, not large enough to make a difference.
I should check the effect of humidity on air mass, too, but from the above calculations, it just doesn't seem worth it.
These effects are much smaller than the ES of even extremely accurately weighed charges.
Cheers,
Keith
Caveat: Dynamic pressure increases with the square of velocity, so hypersonic bullets CAN benefit from light gas, such as helium, in the bore.
Keith, no doubt, I have thoroughly pissed off you, Pacecil, and several others, but I got your attention; didn't I?
Okay,, here we go!!!
First, I must say that I have been through this many times with such notables as Dr. Jack Jackson, the editor of Vaughn's book, "Rifle Accuracy Facts", Jim Borden, who needs no introduction, and Bryan Litz, Chief Ballistician for Berger Bullets. Of these three gentlemen, only one agrees with me; Bryan Litz.
I was unable to discuss this with Harold Vaughn before he passed away, which is a pity because he was a pilot and would have understood what I'm talking about.
I'm not an engineer, I'm a retired airline pilot who spent sixteen years flying small aircraft and sixteen years flying the Boeing 737 for Southwest Airlines. I have a keen understanding of atmospheric conditions, airborne objects, and understand how powerful air can be when compressed.
Keith, in your first paragraph above you said,
"
Gene, I expected someone to counter with the air mass and atmospheric pressure arguments.
You are completely correct that air has to be pushed out the barrel along with the bullet but the effect of changes are so small that the effect is insignificant.
Oh?
Insignificant?
No,,,, the effect is hardly insignificant.
Keith, in your second paragraph you said,
",,,the difference in mass of the air inside a 0.308” barrel 22’ long with a change in temperature from 20C to 30C is about 0.016 grains. Thus the changes caused by the change in air mass should be about the same as that if the bullet were 0.016 grains lighter. That is 0.014% difference for the 118 grain bullets that I shoot. Muzzle velocity, for instance, would go from 3050 fps to 3050.4. Not enough to affect tune."
Keith, here's where you are headed off in the wrong direction; you're thinking of only the miniscule amount of air in the barrel and its weight, not the total weight of the atmospherie which fills every open container.
You are also talking about muzzle velocity and not elapsed time in the bore which is actually what determines bullet exit timing.
If we had to deal only with the miniscule weight of the volume of air contained in the bore, it would indeed be insignificant but that air must be compressed and pushed out ahead of the bullet against the weight of the atmosphere.
And a bullet accelerating down a 22" sealed cylinder from zero to mach 2.7 is one helluva' compressor. The heavier the atmosphere, the greater the drag on the accelerating bullet, the greater the drag the slower the acceleration. Indeed, the bullet may be exiting at the same velocity but have a shorter elapsed in-bore time because of reduced atmospheric drag and the fact it accelerated quicker.
Later
Gene Beggs
