POI Shift

[alabamaslamma]

I recently converted my AR to 6.5 Grendel with a barrel and bolt from Grendel Hunter and took it to the range for the first time today. Once I got past the first 10-20 rounds, the groups tightened up to a reasonable level. Throughout the rest of the shooting session I would take 3 shots within 1 MOA, then each subsequent shot would begin walking down. If I took a break and waited for the barrel to cool down, I’d put about 3 more rounds back in the red zone, followed by another gradual walk down and out.

I prefer this to the random scattershot I had with 224 Valkyrie, but would like to resolve the issue.

Anybody have similar experiences that they’ve resolved and care to share any tips?

[StoneHendge]

Does the dispersion also increase as the POI shifts down?

[alabamaslamma]

Yes, it does.

[StoneHendge]

In my experience, that could be thermal drift. This was a BSF barrel which has a unique form of construction so some sort of warping could also have been occurring, but the relatively thin steel was definitely getting very hot. https://www.65grendel.com/forum/showthre...hlight=BSF

I'm experiencing a similar phenomenon to what you are with a new Odin .223 barrel. After 10 rounds it seems to be hotter than it should and definitely takes longer to cool down than it should. I'm guessing that the steel on the Odin wasn't cured properly. I plan on running the same sort of test that I did with the BSF when I have the time.

[alabamaslamma]

That’s interesting. Weird thing is my barrel isn’t particularly thin - much thicker than an M4A1 barrel, which I’ve seen shift much less(though spread does happen with those). Other people seem to have good results with the GH(Satern, I believe) barrels.

I’m thinking there my be unequal stress somewhere in the system, which shows itself when the metals warm up. I’m heading off for a few weeks for work, but I’m gonna try out my theory when I get back and I’ll post the results.

I’m either going to order a new upper receiver or have a gunsmith lap my current one. I’ll also order a new barrel nut. To tighten up my spread on all shots, I’m going to install a G2S trigger group; my current trigger is a run-of-the-mill, cheap, heavy, single stage which requires serious concentration on each shot to retain any precision. I’ll remove, re-level, and torque my optic carefully.

All that said, I’m no sniper, but I’d know if I moved enough to shoot 3” down at 100 yards, and I know a repeatable pattern when I see one. Hopefully this project saves someone else some time and headache in the future.

[grayfox]

You can lap the front of the upper receiver fairly easily, I used a cut-off barrel end and shaved down the pin.
But more to the point, for the $$ you're planning, I would get a better barrel, grendel hunters are rack grade just a bit better than BCA but a roll of the dice never the less.
Plus the trigger, and for sure the torqueing... but don't get hopes too high for your current barrel.

[alabamaslamma]

Good points. I can lap myself without much trouble, I’m sure. I won’t mind at all if it’s 1-1.25MOA. I’m not after sub-MOA with this one as it’s a hunting and short to medium distance sport shooting project. If it doesn’t perform consistently after the tune-up I’ll hit up grendel hunter customer service. Minor heat-induced spread is fine with me, just not major drift.

[VASCAR2]

Make sure the gas block is not hitting the hand guard under recoil. Ensure the gas tube is centered in the receiver and not in a bind. Check any muzzle device. Over torquing a muzzle device to get proper timing can cause issues. As a last resort shoot the barrel with no muzzle device to eliminate a potential cause.

I’d double check the torque spec on my scope mount to ensure it is not the optic. If I had a spare optic I’d try a different scope to eliminate that possibility.

I have several AR-15’s in 6.5 Grendel and 5.56/223 and have never noticed the groups moving as you described. I have noticed my groups open up when the barrel of my Cz 527 gets hot but the stock needed relieved to give more clearance around the barrel.

[kmon]

I have seen it on several standard bolt actions and free floating the barrel corrected it.

[lazyengineer]

FWIW, and not saying this is your case, but I did get the exact same symptoms. In my case it was an AA 20" HBAR, and it took me a while (and a few boxes), to figure out that the barrel extension wasn't on very tight. They used some kind of Loctite, and when cool, it was super tight and great. But if I put a few round through it warmed it up, it got loose, and the barrel would droop in the receiver and within the free-float handguard. You could wiggle it with your hands. Let it cool down, and it tighten back up enough you couldn't notice, and stopped drooping. I pulled the barrel off, and sure enough, was able to unscrew the barrel extension right off with my hands.

I contacted Alexander Arms, and they did fix it; though the jerks made me pay the shipping to send it back to them. I'm irritated by that, since it was their defective worksmanship, but whatever. They then sent it back and my headspace and timing were still good. Not sure how you can tighten up the threaded barrel extension without affect those things - but they did. It works great now.

[Klem]

Slammer,

Vertical spreads are typical of thin barrels and I have an old 1921 SMLE that does this, but it walks up the target, not down. Stringing up the target is typical so in your case something else is happening but related to heat. You can tighten up the screws etc but the gun already shoots well when it is cool, so that eliminates everything except heat. The free-floating aspect might be the cause but if there is space around the barrel and the only thing connecting it to the rest of the gun is the gas tube then that is as good as you are going to get.

My money is on one of two things; you've got a dud barrel or the mirage above the barrel is messing with how you see the target through the scope. Dud in the sense that your barrel is shifting too much when it heats up. As for barrel mirage this is most noticeable on a calm day and at night when the wind is not helping blow the boil away. In bright light it is there but we don't notice it, more or less. I do a lot of shooting at night when it is calm and it is definitely noticeable, especially with a suppressor. The air boils after about the first 10 rounds. Cranking down the scope and waiting between strings helps. Taking off the suppressor helps with mirage but then you don't get the benefit of milder recoil and gas taming behind the bullet. Bench rest shooters often use louvres above their barrels to deflect the boil from the scope axis. In an AR doing that that is pushing reason and given the very specific stringing mirage is possible but unlikely. Usually mirage just blows groups out or you get one round wide. So, I would do your checks while considering buying a better barrel.

Lazy's situation above could also be the reason, if your extension is glued (gluing extensions is contentious and opinions vary).

[Old Bob]

I have an old .270 Win. Ruger M77 that does that. You can cover the first 3 shots at 200yds with a fifty-cent piece. Shot 4 is an inch right & 2 inches high. Shot 5 is 2 inches right & 5 inches high. It's done that since I bought it new in 1970.