What to think if an AR-15 starts firing bursts on its own...

[AJMD429]

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A patient of mine said she was firing an AR-15 that she purchased new 6 months or so ago and had about 500 rounds through it without problems, when it started to fire two to six shots with one pull of the trigger intermittently. (I always think it's funny that people ask their physician about gun stuff, but she had been sexually assaulted about 20 years ago and I helped her learn handgun safety and connected her with another patient of mine who is a female cop and taught practical carrying tactics self defense shooting for women, so she thinks of me as her answer guy when it comes to guns I guess.)

Anyway, the only thing I could think of would be that a sear or trigger surface has probably chipped. I wouldn't think they would wear after so few rounds unless it was a real problem with the quality. The gun was a PSA and I've never had trouble with any of those personally. I don't know if she's comfortable disassembling the lower stuff, but I told her to look inside with a flashlight and see if it looked like the springs were in the places they should be, comparing to pictures she could surely find on the internet. I doubt she could see the potentially damaged surfaces without disassembling the lower however.

Ultimately she will probably wind up taking it to a gunsmith, but I thought about offering in the meantime if she wanted to just order a lower parts set that would include the trigger and sear and disconnect, she can bring it into the office and I would replace those parts with the new ones and we could look and see if the old ones were chipped. I also told her since the alphabet agencies seem to be more interested in finding technical violators with the wrong political beliefs than they do actual criminals, that she should keep the gun disassembled and get whatever parts are in it out of there as soon as possible. Legal consideration aside, it's also not terribly safe to have a firearm that discharges more times than you intend, even assuming your muzzle is pointed in a safe direction.

Other than a chipped engagement surface or maybe a spring miss assembled or too weak, can you think of any other causes to look for?

[wecsoger]

No, I think you got it, the 'fire control group' is why it's called that.

Obviously take a close look at the trigger/disconnector surfaces and alignment.

First off, before digging in too much I'd stick an eyeball in there to make sure it was correctly assembled in the first place.

Semi auto AR's are rock simple so something has to be obviously wrong when they start doubling, tripling, etc.

I shall expect a full report, with pictures.

[Rockrat]

Check the firing pin to make sure it is not binding and moves freely. Too light a trigger pull can cause a slam fire too, IIRC, so the fire control parts are a good start

[Pisgah]

For sure, something in the fire control group has gone amiss. Personally, I once installed springs that were supposed to lower the trigger pull, which they did -- but also they very neatly converted my normal, semiauto, lousy-trigger AR in to an unpredictable full-auto. Oops.

Rather than attempt to diagnose the precise problem here, I'll say stock fire control group components are cheap and easy to swap out, so I'd just install all brand-new bits.

[Grizz]

i knew someone with an FAL that did that. He smiled every time.

[1894cfan]

Might not be what the problem is.. but I put together a P80 AR lower and the FCG holes were SLIGHTLY out of speck and when I put in the stock FCG parts and took it out to shoot it did pretty much the same thing, off speck full auto. Ended up getting an en-block after market FCG, in my case a Rise Armament Super Sporting Drop-in trigger group with anti-walk pins with the "Don't tread on me" logo on the side, which solved the problem and lightened up the trigger pull some to boot.
So, you might want to check the FCG pin holes while you're checking out everything else. HTH

[jkbrea]

I have a 10.5" PSA upper and Anderson lower. It did the same thing...firing 3-6 round bursts. The hammer was chipped. It was a cheap fix. I just replaced it with a mil spec trigger assembly.

[rossim92]

when you clean an ar , you must have the safety on at all times. when you take off the upper to clean the gun the hammer must not fall in any circumstance, it can damage the trigger group or the lower. that might have happened with the previous owner and the ar just got to the point any damage caused is now showing up. I could be wrong, but just my two cents.. I agree, take it completely apart and check for any chips or cracks.

[Ray Newman]

This thread brought back A memory. In Fall of 1964, when I was in infantry training regiment at Camp Geiger, the Marine Corps issued M1 rifles. At Parris Island, we trained with the M14, but we were issued M1's at Gieger as the Corps reasoned that our allies were issued M1's and someday, someplace, we just have a need to use a M1.

Those old Garands were well used. When we first familiarization fired them, the one I had fired one round, then the rest clip on automatic fire. Armorer came over and gave me another clip, I fired it and it fired full auto on the last 5 rounds, even if without my finger on the trigger. Armorrer pulled it apart and the trigger group was well worn, tossed it in a bucket, put another in and the rifle functioned normally. Talk about a big surprise. Those old GI .30 M2 rounds were authoritative on both ends!

[Rimfire McNutjob]

Disconnector not catching.