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.
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.
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).
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.
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.
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.
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?
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.