12.5 AR pistol not grouping

[a1madrid]

I just did a 12.5 build and ran into an issue at the range, but first let me tell you what parts are in the gun. I used a SOLGW 12.5 carbine length gas system barrel with 1/7 twist, Silencerco ASR flash hider, SOLGW lo profile gas block, SOLGW M76 10.5 inch M-LOK rail, Aero upper with Radian SD charging handle, aero lower, aero lower parts kit with a Geissele SSA-E trigger, VLTOR A5 buffer tube with a Sprinco green spring and the VLTOR A5H2 buffer, and running an SB tactical SBA3 brace. I'm running a Trijicon ACOG TA31F-G on the top that I made sure I mounted correctly and it is snug on the rail. My issue is I took it out to the range 2 days ago and with 55 grain 5.56 American Eagle, at 100 yards it was ALL OVER the page. Like no consistency or grouping whatsoever. One shot would be low 3 inches and the next would be high and right 7 inches, and then just randomly throwing shots all over the page 6-10 inches apart from each other. It was literally 6-10 inches between each shot with no grouping whatsoever it was just randomly putting holes all over the page. I've zeroed several ARs with 1/7 twist with this exact ammunition and have never gotten a result like that. As a matter of fact my 16 inch Daniel Defense barrel groups this exact ammo at 1.5 MOA at 100 yards.
I'm assuming it's not the ammo and that I may have done something wrong when building the gun. The M76 wedge lock rail I bought, I did not see any instructions for how tight to tighten the barrel nut so I BELIEVE I tightened it to somewhere in the 45-50 foot pounds range. I was told that the barrel nut could be too loose but I don't know. Maybe I tightened it down wrong. Do you have any suggestions on what the issue could be? I am guessing I did something wrong when building the gun because I don't know what else would cause such inconsistent accuracy. There was no repeatability from shot to shot no grouping whatsoever just all over the page. What do you suggest? Thank you. Sorry for the long post I just really want to get this gun squared away.


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[kirkland]

Did you use a torque wrench on the barrel nut?

[a1madrid]

Did you use a torque wrench on the barrel nut?

Yes I did. I used 45-50 foot pounds of torque.


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[grizzman]

The barrel nut is definitely not too loose. Even if it's a few ft lbs too tight, that shouldn't cause 6-10" groups. My first guess is that the issue is with the optic or its mount.

How are the groups with irons, or a different optic? How are the groups with the DD 16" with the ACOG installed?

While rifles have their preferences with ammo, this performance seems well beyond what could be reasonably expected even if the barrel absolutely hates that particular ammo.

[Coal Dragger]

Try a different optic that you know to be in good condition. Then try some different ammunition, preferably a known high quality match load.

Also try formatting your post with paragraphs. Makes it a lot easier to read that way.

[constructor]

I just did a 12.5 build and ran into an issue at the range, but first let me tell you what parts are in the gun. I used a SOLGW 12.5 carbine length gas system barrel with 1/7 twist, Silencerco ASR flash hider, SOLGW lo profile gas block, SOLGW M76 10.5 inch M-LOK rail, Aero upper with Radian SD charging handle, aero lower, aero lower parts kit with a Geissele SSA-E trigger, VLTOR A5 buffer tube with a Sprinco green spring and the VLTOR A5H2 buffer, and running an SB tactical SBA3 brace. I'm running a Trijicon ACOG TA31F-G on the top that I made sure I mounted correctly and it is snug on the rail. My issue is I took it out to the range 2 days ago and with 55 grain 5.56 American Eagle, at 100 yards it was ALL OVER the page. Like no consistency or grouping whatsoever. One shot would be low 3 inches and the next would be high and right 7 inches, and then just randomly throwing shots all over the page 6-10 inches apart from each other. It was literally 6-10 inches between each shot with no grouping whatsoever it was just randomly putting holes all over the page. I've zeroed several ARs with 1/7 twist with this exact ammunition and have never gotten a result like that. As a matter of fact my 16 inch Daniel Defense barrel groups this exact ammo at 1.5 MOA at 100 yards.
I'm assuming it's not the ammo and that I may have done something wrong when building the gun. The M76 wedge lock rail I bought, I did not see any instructions for how tight to tighten the barrel nut so I BELIEVE I tightened it to somewhere in the 45-50 foot pounds range. I was told that the barrel nut could be too loose but I don't know. Maybe I tightened it down wrong. Do you have any suggestions on what the issue could be? I am guessing I did something wrong when building the gun because I don't know what else would cause such inconsistent accuracy. There was no repeatability from shot to shot no grouping whatsoever just all over the page. What do you suggest? Thank you. Sorry for the long post I just really want to get this gun squared away.


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Take a close look at the crown.

[a1madrid]

Take a close look at the crown.

This might sound like a stupid question but how do I look at the crown with a muzzle device on there? Take it off and inspect the crown of the barrel?


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[rvanno]

First and easiest thing to do is try different ammo.

[a1madrid]

Try a different optic that you know to be in good condition. Then try some different ammunition, preferably a known high quality match load.

Also try formatting your post with paragraphs. Makes it a lot easier to read that way.

I had originally thought different ammo but the optic is a brand new ACOG that’s mounted solid on a Larue QD mount.


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[grizzman]

The optic's newness makes it more likely to be the cause than if it has been in use without issue for 2 or 20 or 40 years. Is the LaRue mount the lever variety? If so, how much tension do you have on it? I set mine to be almost unremovable without a loop of paracord to open the lever.

I highly suggest putting whatever optic onto the pistol that you have been using to get 1.5 MOA groups with the DD.

Basic troubleshooting isn't difficult....and a great excuse to need to go to the range.

.....and yes, removal of the muzzle device makes it MUCH easier to view the crown (unless it's pinned).

[georgeib]

One more issue to consider is how tight the muzzle device is. There's a video floating around of a guy using a muzzle wear gauge showing how much different torque settings constrict it. It's surprising to see how little torque it takes to change how far the gauge goes in.

[Coal Dragger]

Yeah 20-30ft-lbs is all you want or need. The closer to 20ft-lbs the better in my limited experience. I usually use a shim kit, since I don’t like crush washers. I’ve gone so far as to put a shim on a whet stone to slowly grind it down until I hit my desired torque value.

Then off to the range to make sure everything is happy and that I like the muzzle device. Afterwards I will pull it off, clean/degrease, and apply Flexbar Rocksett. You don’t need a bunch of gorilla level torque to secure it with Rocksett.

[Coal Dragger]

I had originally thought different ammo but the optic is a brand new ACOG that’s mounted solid on a Larue QD mount.


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OK.

New optic, new carbine, new mount.

You need to start changing variables there that you can control for. So again start with an optic and mount you know from prior experience are functional and hold zero. If you have one it’s time to try it out.

Your issue sounds more like an optic/mount issue than anything else. I am always skeptical of throw lever QD scope mounts, and suspect your issue lies there. If you can’t trust the optic enough to use a bolt on mount that actually stays put, then it shouldn’t go on the gun at all in my unsolicited opinion.

[Todd.K]

Scope and mount are the first thing to suspect.

You need to torque the barrel nut to max and loosen a couple of times before finish torque.

[constructor]

This might sound like a stupid question but how do I look at the crown with a muzzle device on there? Take it off and inspect the crown of the barrel?


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Yes, with magnification. look for burrs or the chamfer being off center.

[a1madrid]

The optic's newness makes it more likely to be the cause than if it has been in use without issue for 2 or 20 or 40 years. Is the LaRue mount the lever variety? If so, how much tension do you have on it? I set mine to be almost unremovable without a loop of paracord to open the lever.

I highly suggest putting whatever optic onto the pistol that you have been using to get 1.5 MOA groups with the DD.

Basic troubleshooting isn't difficult....and a great excuse to need to go to the range.

.....and yes, removal of the muzzle device makes it MUCH easier to view the crown (unless it's pinned).

Yes mine is the Larue QD mount with a lever. When I adjust the tension of the mount do I just tighten the nut that is on the side with the locking lever that locks it into place or do I tighten both of them?


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[SteveL]

I agree with everyone else here. Optic and/or mount are the 1st variables I would try to eliminate, followed by barrel nut torque (which you've already accounted for) and muzzle device torque.

[grizzman]

Each locking lever has its own tension adjustment nut, which is clearly visible from above the mount.

The mount came with a wrench to adjust them.

[Coal Dragger]

A scope mount that needs adjustment to work right is a scope mount that shouldn’t be used.

A simple Geissele mount for example with the nuts torqued to 65-70in-lbs isn’t going anywhere and isn’t going to throw zero off all over the place.

[grizzman]

I’ve never had problems with any of my QD mounts. I bet he vast majority of all other users that installed them properly have had the same result.

Most of them need to be adjusted the first time they’re installed, and never again.

I run non-QD on bolt guns, but not ARs.

[MegademiC]

A scope mount that needs adjustment to work right is a scope mount that shouldn’t be used.

A simple Geissele mount for example with the nuts torqued to 65-70in-lbs isn’t going anywhere and isn’t going to throw zero off all over the place.

Geissle is not qd.
Larue mounts work very well when used properly, and dont lose zero.

[bamashooter]

Likely the optic, mount, etc. Simply remove the assembly or actually no need.

Using a bench and spt while maintaining a steady hold, simply point and fire the gun at any target at your preferred distance. once you have created a point and shoot aiming point, continue to fire a few. If it's optics-related, it should have an acceptable group since optics were removed from the equation. Probably mentioned. Make sure mount and optic are positioned fully forward in their rail slots when installing.

[masenomics]

Like others have mentioned I’d suspect it’s the muzzle device. I over torqued a muzzle device once and the end result was much like yours would not group and shot sporadically in a roughly 8-10” circle, luckily it was just a bushmaster barrel.

[Cameron]

Like others have mentioned I’d suspect it’s the muzzle device. I over torqued a muzzle device once and the end result was much like yours would not group and shot sporadically in a roughly 8-10” circle, luckily it was just a bushmaster barrel.

Does anyone know what is happening when you over torque the muzzle device? Does it mess the crown or something?

I can't think how over tightening a muzzle device would make for inconsistencies, moving POI but can't imagine how it would be inconsistent.

Cam

[SteveL]

Does anyone know what is happening when you over torque the muzzle device? Does it mess the crown or something?

I can't think how over tightening a muzzle device would make for inconsistencies, moving POI but can't imagine how it would be inconsistent.

Cam

I'm not sure the actual mechanics of it, but I've read from reliable sources on this board (I'm pretty sure C4IGrant was one of them) that they have been able to improve a rifle's groups by removing/reinstalling a muzzle device that had been installed with a lot of torque on it.

[Coal Dragger]

Geissle is not qd.
Larue mounts work very well when used properly, and dont lose zero.

I know a Geissele mount isn’t QD, instead it’s nearly bomb proof, simple, and solid.

For the record I am just not a big fan of QD mounts.

[Coal Dragger]

Does anyone know what is happening when you over torque the muzzle device? Does it mess the crown or something?

I can't think how over tightening a muzzle device would make for inconsistencies, moving POI but can't imagine how it would be inconsistent.

Cam

Over torquing the muzzle device can deform the bore enough to cause issues. Fortunately pulling it off and re-installing to specified torque will usually allow the barrel steel to return to its normal dimensions and then perform as intended.