New ammo batch - 3 MOA shift left

[lsllc]

Serious question (I'm not a reloader): What about a powder change would make a bullet skew left or right? Sure, a minute amount but 3-4 MOA? I can easily digest the up or down variance as it relates to velocity (hotter batch vs. not as hot). Wouldn't there have to be a sizeable outlier (like burr in the barrel, slightly-off muzzle device that "bumps" the bullet, loose or re-mounted optics) in the equation for windage discrepancies like that?

Barrels are like a giant tuning fork and whip up and down as the explosion travels down the barrel. This begins at the muzzle and as the bullet exits. Additionally, turbulence is applied to the bullet from pressure exiting the muzzle.

Changing powders to something with a different pressure and burn rate can cause the barrel to whip in a different way and provide different turbulence acting on the billet. When the whip is at a different part of the wave as the bullet exits, the point of impact shifts. Remember, if only takes a minor variance at the muzzle to result in a major difference downrange. It literally only takes a few thousands of an inch difference to show up at 100.

When you change both powder AND bullets, which seems to be the case with the OP, larger shifts are more likely to manifest. When changing powder from H4831SC to H4350 in my competition rifle, at the same velocity, I have nearly a 2 MOA shift. This is with a heavy heavy match barrel which is capable of sup half minute precision. The less precise a barrel is machines, and the more axial misalignment in the system, the more likely a significant shift is going to happen. It isn’t uncommon for ARs to exhibit a large degree of axial misalignment.

Controlling the “whip” is what load tuning seeks to accomplish. This is why handloaders seek nodes. A node is essentially a plateau in the pressure curve which will provide both consistent velocity and barrel harmonics. Harmonics, or barrel whip, is what influences precision (consistency shot to shot) while consistent velocity is what allows predictable trajectory curves.

Here is a video that demonstrates barrel whip. It is random from YouTube and I cannot take credit for the video.


https://m.youtube.com/watch?v=hru_sL7BFN0

Sent from my iPhone using Tapatalk

[gaijin]

^^ Which is why when changing components/lots we are advised to prove at the bench and tweak as necessary, to obtain desired results.

[masan]

I have, on two different occasions, gotten to a match to find that my windage or elevation got changed when cramming my rifle into the cutout in the foam of my Pelican case.

Any chance that was the issue?

Would be a big time saver/headache reliever to find out it was something simple rather than a tune issue with your pet load.

[markm]

I've seen this on my buddy's .308. He switched nothing except bullets (AMAX in place of SMKs) and go a 3MOA shift left. Both shot good, but the AMAX moved left 3 inches.

[Bimmer]

Barrels are like a giant tuning fork and whip up and down as the explosion travels down the barrel.

And they whip side-to-side, too...