New ammo batch - 3 MOA shift left

[Eurodriver]

Is this common?

I switched to a new batch of ammo (powder and bullets) and I was 3 MOA left. The wind was crazy (16mph) changing directions every few minutes and I dropped about 15 rounds and 2 stages before I figured out what happened. I’ve never had this happen before. Why the big shift?

[gaijin]

Not in my experience. Assuming handloads?


At what distance was the 3 MOA shift from prior load observed? Extreme range, 600 yds and out or from a couple hundred and out? And was the 3 MOA differential elevation, windage or both?
And we are talking different lots of the same bullets and powder?
Depending on specific powder, I’ve had some fairly extreme differences in pressures/velocities, not as severe with bullets of same type/manufacturer.
I have blended 8 pound kegs of different lots together to avoid this.

I would’ve “proven” the new batch prior to a significant match.

[Eurodriver]

It was actually 4 MOA left and 0.5 MOA low. I have never seen this before and I’ve used this load for years. I noticed it at a stage with 323 yards and 416 yards. Before the next stage I ran over to 100y and confirmed it since the wind was stupid.

[gaijin]

What caliber Euro?

A .224 pill, even heavy, drifts like a bitch in wind.
You have 15/20 mph wind drift plugged in ballistic program and it exceeded that?

[Eurodriver]

It was definitely not wind because I moved over to a different range and shot at 100y with high berms. Maybe 5mph of wind made it down there.

.223. I chrono’d the old load yesterday at 2746 77gr TMK. I didn’t chronicle this load though yet

[ABNAK]

Not assuming it as a given, but is it the same weapon?

Elevation could obviously be a charge/velocity issue, the windage (since you confirmed it wasn't the gusts that were affecting it) is something else.

Optic or mount? Some kind of barrel, muzzle, or muzzle device defect causing a deflection to the left? At those ranges the "spin drift" shouldn't be an issue, and if it was it would be to the right due to the RH twist (correct?).

[gaijin]

Interested to hear how the new batch chronos.

[lsllc]

I have witnessed different powders/bullets cause quite a shift. Usually it’s not as drastic as 4 MOA but it isn’t out of the question.


What was the old load? What was the new load?


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[opngrnd]

I have seen .5gr shifts in weight with my powder thrower when going from one powder lot to another. I was using TAC when that happened, and TAC has long worked well for me as an accurate plinking load.

[ABNAK]

I have witnessed different powders/bullets cause quite a shift. Usually it’s not as drastic as 4 MOA but it isn’t out of the question.


What was the old load? What was the new load?


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?

[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

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[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...