Need help! My AR started doing "2 round burst"

Dieseltech09 · Sep 4, 2011

So 1st what did I do to my AR? This weekend I changed my buffer tube to a mil-spec. Keep the factory Buffer and spring (DPMS) and installed a set of KNS anti-rotation pins. just pushed them through. I didnt have the trigger group out. Thats it.

My brother and I were out shooting, shot a mag everything was fine. on the second mag after a couple of rounds I thought i felt a funny recoil. Had my brother shoot it so I could watch what was happening. Sure enough about every 6 - 10 rounds it would double fire. I have looked and cant see anything out of place. It needs to be cleaned but its not that dirty. I was also using some new P-mags that also weren't catching the bolt on the last round for some reason.

Any ideas?

DoctorJJ · Sep 4, 2011

In my experience, it's usually been a disconnector issue where it isn't catching the hammer and you get a "jar off" double. It can be a few things though.

DoctorJJ · Sep 4, 2011

Take the upper off, catch the hammer with your finger as you pull the trigger. Keep the trigger pulled and then reset the hammer. After it's reset, flip it down and wiggle it, pull up on it, etc. If it comes loose from the disconnector and slams forward then it likely confirms that it's jarring off the second round.

Another possible scenario, if you have an aftermarket trigger and you have very little over travel and it's set very light, your finger can be "jarring off" the second round as the gun recoils in your hand.

A negative sear engagement angle could cause this too but I haven't personally ever seen that with unmodified parts. You or someone else didn't try to do a trigger job before???

imacgyver · Sep 4, 2011

figure out how to make it a 3 round burst and you got yourself one of them real mil-spec ones! Two round burst could be awesome in a 3 gun match.

+1 on the Dr. JJ post

Dieseltech09 · Sep 4, 2011

Ill check that out. No aftermarket trigger, whatever LPK DPMS uses is whats in it.

Dieseltech09 · Sep 4, 2011

JP aka iMacGyver said:
figure out how to make it a 3 round burst and you got yourself one of them real mil-spec ones! Two round burst could be awesome in a 3 gun match.

+1 on the Dr. JJ post

I wouldnt have a problem with that I just dont want to to go full off when Im not expecting it or while my wife is shooting it.

imacgyver · Sep 4, 2011

have you inspected the firing pin as well?

Wall · Sep 4, 2011

Brian check the old & new pins with a caliper.
The pins should not be super easy to push in.
.154 pins can't be used to replace .156 pins.
A buddy's Colt lower did this when he unknowingly replaced the large pins with small pins.
He ended up having to order pins from KNS to fit his lower.
Swap the pins back & see if it still happens.

Dieseltech09 · Sep 4, 2011

After resetting the trigger its takes a pretty hard pull but I can pull it off the disconnector. So far no amount of wiggling or bumping has made it move

Dieseltech09 · Sep 4, 2011

Wall said:
Brian check the old & new pins with a caliper.
The pins should not be super easy to push in.
.154 pins can't be used to replace .156 pins.
A buddy's Colt lower did this when he unknowingly replaced the large pins with small pins.
He ended up having to order pins from KNS to fit his lower.
Swap the pins back & see if it still happens.

Wall, the KNS pins i bought are .154 I dont have a caliper here to measure the old ones. Ill have to wait till I go back to work Tuesday to measure them. Thanks for the heads up

TroyF · Sep 4, 2011

Brian, holler at Weathers out at your employers range. He should be able to help. Or Jason Stearns.

Dieseltech09 · Sep 4, 2011

Thanks Troy. Weathers has helped me out before with a few things. Hes a great guy and I always enjoy chatting with him. Ill give him a call tomorrow

DoctorJJ · Sep 5, 2011

When you recock the hammer, while holding the trigger back, push the hammer down as far as you can and let it go. Flip it like that a few times and see if it slips past the disconnector. You might try taking it apart and making sure a piece of grit didn't get under the front of your disconnector.
I don't think you should be able to pull it and get it free of the disconnector. May need to hit the front of the disconnector with a stone a few passes to advance it slightly. If you decide to do this, go ever so slowly and reassemble checking each time. You can't put metal back.