AR10 Acceptable Accuracy (10 Shot Groups)

[eastexsteve]

Are you saying bolt speed is fixed regardless of how much gas is used to cycle bolt?

Of course it isn't. But, if you are saying that bolt speed is controlled by the pressure exerted by the number of rounds in the magazine, then you have to acknowledge that pressure constantly changes as the magazine empties and the mag spring reduces tension. I would think that other issues like case pressure have much more influence over bolt speed. Definitely not scientific, but one way you tell if a gun is or isn't properly gassed is where it throws the empty cases. And, where your gun might be properly gassed for one load, if you drastically change that load, it might no longer be properly gassed and need to be adjusted. But, if your loads are constant, then it's going to throw them in almost the same spot. However, I can definitely change where it throws them by significantly changing the load/ammo. But, the entire magazine load will still get pitched in the same spot, as long as they are the same load, generating the same case pressure. At least in my guns, the first round from the magazine will hit the ground close to the same spot as the last.

And, regarding bolt speed, what do you think the speeding bolt is doing to the bullet? Is it causing the bullet to move forward in the case depending upon its speed when it slams it into battery? Do you think it is somehow deforming the case to some degree as it slams it into battery, depending upon the speed of the bolt? If either of these is true, then you likely have other issues to deal with other than the speed of the bolt.

Just to compare, I have an HK91. I have yet to see ANY gas gun slam a bolt harder into battery than that gun. Whether from a locked open position, or during firing, it slams the crap out of a round. And, when it comes out of battery to eject a round, it slams back so hard that when it hits the ejector, it will throw the empty case 25 feet. It will throw it so hard that if it hit you in the head, you would likely be bleeding. Yet, as hard as it hammers the bolt, I have yet to pick up a deformed case. I have also measured loaded rounds to see if the bullet is moving forward in the case during battery. They don't. I also notice no damage or marking on the face of the rim of the case. It's not a "sub-MOA" gun. But, from a vise, the grouping doesn't change from round one to round twenty as long as I'm using ammo it likes.

 

[FCS]

Of course it isn't. But, if you are saying that bolt speed is controlled by the pressure exerted by the number of rounds in the magazine, then you have to acknowledge that pressure constantly changes as the magazine empties and the mag spring reduces tension. I would think that other issues like case pressure have much more influence over bolt speed. Definitely not scientific, but one way you tell if a gun is or isn't properly gassed is where it throws the empty cases. And, where your gun might be properly gassed for one load, if you drastically change that load, it might no longer be properly gassed and need to be adjusted. But, if your loads are constant, then it's going to throw them in almost the same spot. However, I can definitely change where it throws them by significantly changing the load/ammo. But, the entire magazine load will still get pitched in the same spot, as long as they are the same load, generating the same case pressure. At least in my guns, the first round from the magazine will hit the ground close to the same spot as the last.

And, regarding bolt speed, what do you think the speeding bolt is doing to the bullet? Is it causing the bullet to move forward in the case depending upon its speed when it slams it into battery? Do you think it is somehow deforming the case to some degree as it slams it into battery, depending upon the speed of the bolt? If either of these is true, then you likely have other issues to deal with other than the speed of the bolt.

Just to compare, I have an HK91. I have yet to see ANY gas gun slam a bolt harder into battery than that gun. Whether from a locked open position, or during firing, it slams the crap out of a round. And, when it comes out of battery to eject a round, it slams back so hard that when it hits the ejector, it will throw the empty case 25 feet. It will throw it so hard that if it hit you in the head, you would likely be bleeding. Yet, as hard as it hammers the bolt, I have yet to pick up a deformed case. I have also measured loaded rounds to see if the bullet is moving forward in the case during battery. They don't. I also notice no damage or marking on the face of the rim of the case. It's not a "sub-MOA" gun. But, from a vise, the grouping doesn't change from round one to round twenty as long as I'm using ammo it likes.

You really misconstrued comments.

First round chambers differently because of bolt speed.
Last round is different as no cartridge pressing on bottom BCG.

I'm done tilling an infertile field.

 

[eastexsteve]

You really misconstrued comments.

First round chambers differently because of bolt speed.
Last round is different as no cartridge pressing on bottom BCG.

I'm done tilling an infertile field.

You've yet to explain how the first round chambers differently due to bolt speed. If the bolt locks up with the barrel, then it locks up. Either the differing bolt speed is causing some deformation, damage, or change to the round of some sort, or it isn't. Something about the round itself has to be changing to cause a significantly different impact point. And, if you've got such a sloppy fit between the bolt lugs and barrel with a loaded round that any kind of pressure (or lack thereof) on the bottom of the bolt causes the round to significantly move in the chamber, that appears to be a headspace issue.

 

[rlsmith1]

Would a property headspaced bolt make a difference here? It seems that guys don't have this problem (at least to this degree) in the AR15 platform and I wonder if that's because there is one standard for the AR15.

I'm actually looking at upgrading my barrel at some point (another discussion, but I'm leaning Criterion) and I'm weighing the cost/benefit of a headspaced bolt matched to the barrel. In my mind, an out of spec (or on the upper end of the tolerance spectrum) bolt will be more impacted by other changes like bolt speed and friction on the carrier. I'm not 100% sold on these even having an impact, but it does seem that having a headspaced bolt could definitely bring repeatability back into a system with (inherently) more tolerance stacking than an AR15.

 

[eastexsteve]

Would a property headspaced bolt make a difference here?

Headspacing has to do with the bolt, chamber, and round. But, to answer your question, compared to improper headspacing, you bet it would. And, especially in a .308win / 7.2x51 gun.

It seems that guys don't have this problem (at least to this degree) in the AR15 platform and I wonder if that's because there is one standard for the AR15.

Yes, they have this problem. "Go" and "No go" gauges have a purpose.

I'm actually looking at upgrading my barrel at some point (another discussion, but I'm leaning Criterion) and I'm weighing the cost/benefit of a headspaced bolt matched to the barrel. In my mind, an out of spec (or on the upper end of the tolerance spectrum) bolt will be more impacted by other changes like bolt speed and friction on the carrier. I'm not 100% sold on these even having an impact, but it does seem that having a headspaced bolt could definitely bring repeatability back into a system with (inherently) more tolerance stacking than an AR15.

I went back and looked at your original post. Your talking about instances of sporadic 2 MOA shots mixed in with .5 MOA shots. Any little thing could cause this. (Including the shooter.) And, as good as FGMM is, it can have flyers. I've encountered other factory ammo that shoots as good or BETTER than FGMM in some guns. Does the gun have a good, tight .308win target chamber? Or, will it also shoot 7.62x51? I have three 308win guns with tight chambers that won't even chamber 7.62x51 rounds.

Before shooting again, make sure the bolt face is clean, and make sure the bolts and barrel lugs are good and clean. Make sure the relief behind the barrel lugs where the bolt lugs lock into is good and clean. Make sure the chamber is clean, but don't clean the barrel. A sure way to come up with an odd shot is to send it down a clean barrel. Take your calipers to the range with you and measure the overall length of the rounds you're shooting. See if they are all consistent. Of course, as the barrel heats up, things could change. In the 100 degree heat I'm having down here, I'm currently trying to dope ammo in a friends gun, and it's a real nightmare. I've had to limit my shooting hours to before sunup and right at sundown. If I had to take the time to drive to a range, It would be a real pain.

 

[billymagg]

Pretty well defined and established phenomenon.
First rd chambers differently than subsequent rds.
Last round has no pressure on bottom of BCG and different mechanical movement.
More pronounced in some rifles than others.
Name of the game is “the same”, first and last rounds are definitely not the same on how sent up feed ramp or pressure on bottom of BCG.
Isn’t like first or last is 2-3 MOA out, pretty common for it to be 0.25”-0.625” out of core group @
100.

IF you have never seen this, couple possible reasons:
You’ve only had exceptional rifles.
You’ve seen this but don’t recognize it as outlier shot(s) are within your normal group size.

Yes, but you'll get a lot of whining about the first round flyer and the last round flyer. All magazine fed weapons are highly dependent on individual magazines. It boggles the mind how many different variables we find with our 308 ARs.

 

[rlsmith1]

Quick update...

1. I swapped out the trigger for a lighter, more consistent trigger.
2. There was no gap between the handguard and upper receiver so I remedied that.
3. After reading about best bipod practices, I decided to forego that entirely and shot from bags today.
4. In my opinion, 10 round groups are a testament to both equipment and the shooter.

I shot 3 cold bore shots (150gr CoreLok) that stacked well under 1 MOA. I know, 3 shot group but these were warm up shots and they were much closer together than the 3 closest shots with the same ammo before.

High hopes for the M118LR I had. Sure enough, all 5 shots went well under 1 MOA. I didn't measure but I would guess it was a 0.8 or 0.9 group.

Sounds like a lot of this was shooter induced which was cured by shooting off bags and swapping to a different trigger. I'm not sure if the handguard gap made much a difference (I'm sure it helped) but it's interesting to see.

I would attribute most of (my) inconsistencies with the trigger (requires better form) and the bipod (still learning how to load consistently).

I'll see if I can replicate those smaller groups next time I'm out. In the mean time, any advice on shooting a gas gun from a bipod would be greatly appreciated! I have a feeling my inconsistencies stem from that.

 

[eastexsteve]

Quick update...

High hopes for the M118LR I had. Sure enough, all 5 shots went well under 1 MOA. I didn't measure but I would guess it was a 0.8 or 0.9 group...

I'll see if I can replicate those smaller groups next time I'm out. In the mean time, any advice on shooting a gas gun from a bipod would be greatly appreciated! I have a feeling my inconsistencies stem from that.

I'm not trying to spoil your success, because it looks like you are having some. But, here's how my stock-out-of-the-box M1A National Match iron sight gun shoots M118LR. This is shooting from the shoulder off of one pack at 100 yds:

Because it is shady and dim where I shoot, I have to shoot at a 4" black circle to even see it through those National Match sights. It's hitting where I want it to hit: about 2 MOA high. This gun will shoot Lake City and IMI about the same. The M80 round was the first one out of the magazine. That's where it usually goes. The far left hit was number 2, and the stacked hits were 3 and 4. There were 6 more rounds left in the magazine. Would that gun shoot tighter than this from a vice or sled? You bet it would. But, the events I shoot require shoulder firing.

Is your gun chambered to shoot both .308win and 7.62x51? Or, does it have a .308 match chamber like both my AR10 guns? I would suggest staying with the bags and away from the bipod until you eliminate any other gremlins. I just wouldn't add any more ingredients just yet. If all things are working as they should, you should be able to get down close to .5 MOA at 100 yds over the bags with your guns' favorite ammo.

 

[308pirate]

If it's a 1-10" twist barrel, the 175 GMM should perform better than the 150s.

There is absolutely no evidence of that. None.

The influence of rifling rate on accuracy is vastly overstated. Once rifling rate and muzzle velocity are fast enough to stabilize a bullet, its influence on accuracy pretty much stops until you go waaaaay the fuck too far to the other extreme.

And 1 in 10 is nowhere near too fast for 150 grain-class 30 cal match bullets.

 

[eastexsteve]

There is absolutely no evidence of that. None.

Sure there is. Start getting past 500 yards and greater. But, at 100 yards, likely no difference.

 

[308pirate]

Sure there is. Start getting past 500 yards and greater. But, at 100 yards, likely no difference.

Please entertain us all by explaining how you isolated twist out of all the other variables that affect accuracy.

 

[eastexsteve]

Please entertain us all by explaining how you isolated twist out of all the other variables that affect accuracy.

Why don't you go do your own homework and do some some shooting from somewhere else besides a computer chair. I never isolated twist out of the variables. I specifically stated "performance."

 

[308pirate]

Why don't you go do your own homework and do some some shooting from somewhere else besides a computer chair. I never isolated twist out of the variables. I specifically stated "performance."

I've done more than my share of long range shooting with lighter for twist bullets. That's why I can call BS when I see it.

 

[eastexsteve]

I've done more than my share of long range shooting with lighter for twist bullets. That's why I can call BS when I see it.

Good for you. Nobody was arguing twist rate except you. I was arguing performance. If you want to keep shooting 150gr match bullets at long ranges, then have at it. However, there are much better match bullets (like 175s) for long ranges. I think even the US military figured this out long ago.

 

[308pirate]

Nobody was arguing twist rate except you.

If it's a 1-10" twist barrel, the 175 GMM should perform better than the 150s.

Performance conditional to rifling rate. Yea it was you who brought that up. Not me.

Nonsense

 

[eastexsteve]

Performance conditional to rifling rate. Yea it was you who brought that up. Not me.

Nonsense

I didn't bring it up, the OP did. But, since twist rate doesn't seem important to you, are you going to now claim that a 1-12 twist will shoot a 185 match bullet better than a 1-10 will at .308win velocities?