AR started firing on trigger reset and went FA a few times today

[usmcvet]

My AR 9mm started firing on trigger reset and went FA a few times today.

The lower is an Angstadt Arms 0940 that takes Glock magazines. It has a 9mm 5.5" TROS three lug bbl and a Silenceco Octane 45 suppressor. I shot it a bunch of times with a 16" Colt upper with no suppressor and didn't have any issues after hundreds of rounds. Today I ran about 100 rounds through it. It started shooting on trigger reset and I had some double feeds. It has a Spikes ST-9X buffer and I don't remember the spring weight. I think it's a Wolf xtra power carbine spring. The trigger is an ALG Quality Mil-Spec Trigger (QMS). I just emailed ALG.

I just did function tests here at home and it fires on trigger reset ever other to every fourth time assembled. It only fired once with the upper off on trigger reset. I haven't taken the trigger out yet. Would a broken trigger pin cause this?

Any ideas?

I just tried functions tests again and can get it to fire on release every time if I release the trigger very slowly.

[dmd08]

I'd look at the disconnector. I had a trigger group repeatedly fail function tests the same way as yours. It was a bad disconnector. Swapped a new one in and it was fine.

[usmcvet]

Thanks. That would be an inexpensive fix too!

[Cagemonkey]

If its firing on trigger release it is most likely the Hammer or Trigger sear surfaces that aren't in spec. The disconnector is working because it is catching the hammer and the weapon is firing on the trigger release.

[Code636]

If its firing on trigger release it is most likely the Hammer or Trigger sear surfaces that aren't in spec. The disconnector is working because it is catching the hammer and the weapon is firing on the trigger release.

Something like that on mine. The disconnector spring was actually upside down. The fat side goes in bottom and snaps into place. Disconnector was hitting the spring and not collapsing so when you took pressure off it would release.

[usmcvet]

Thanks guys. I'll take a look at it Monday when I can get my hands on some spare parts at the LGS.

[Clint]

Please see this thread by Ned.

The disconnector needs to be replaced or some material removed from the underside front part to achieve more sear engagement at handoff.

https://www.m4carbine.net/showthread...lease-triggers

[usmcvet]

Please see this thread by Ned.

The disconnector needs to be replaced or some material removed from the underside front part to achieve more sear engagement at handoff.

https://www.m4carbine.net/showthread...lease-triggers

Thanks Clint I will read it now. I did several searches. The only thread I found here was locked!

Well I just searched again and couldn't find the locked thread but found a post of mine from 2010. I didn't remember having the issue then with a different lower. But I did! And damn it I don't remember what happened with that upper either.

https://www.m4carbine.net/showthread...y-firing/page2

[bp7178]

Disconnector, disconnector spring. I'd replace both and NOT start filing on parts.

[Wayne Dobbs]

The "best practice" I teach in the Colt Armorer Course when disconnector related failure/fire on release rears its head is to replace the fire control group in total: hammer, trigger and disconnector. You can try to troubleshoot the thing component by component, but unless you have the time, the range (for test firing) and the IN SPEC parts, you're possibly going on a long journey.

I just reread your original post and noted you had an ALG FCG in the gun. Those are normally very good, which makes me wonder if your lower receiver is out of spec with regard to the FCG pin holes dimensions and/or location. If you drop another quality FCG in there and it doesn't remedy the problem permanently, you have a bad lower.