Accidental full auto from ar-15 9mm

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Greywolf

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I have a registered SBR 9mm AR-15. Built on a regular semi-auto lower, with semi-auto parts. 8.5" barrel, using either modified UZI mags with welded-on block, or unmodified STEN mags. A few weeks ago it started to do 2-3 shot bursts. I thought it might be the trigger (I had an RRA 2-stage NM trigger) so I put the normal trigger pack back in and it still is doing it. Not all the time, but at least once every 30 round mag.

Any idea what this might be? Could it be the firing pin sticking? I have a Magpul M-93B collapsible stock on it with the proper spring and buffer. It is straight blowback (as are most pistol caliber uppers) and has no gas tube/block.
 
I doubt that the firing pin is sticking, but you could try cleaning the bolt again. It sounds like you have done the right stuff, like changing the trigger group. Check the disconnector spring. Sounds like it is not holding the hammer strongly enough> If that doesn't help, you may need to find a good AR gun plumber to work it over.
 
Another person on another board suggested a heavier CAR buffer in the stock. Said he was having the same problems with a 9mm AR with a CAR stock (I have the Magpul CAR stock) and putting a heavier buffer in it solved the problem. Apparently the lighter buffer he had in it was throwing the bolt back so hard that it cased the hammer to fall again.
 
Had to go to the book on this one. Not knowing which parts of the trigger assembly you swapped out, the suspect parts are your disconnector, hammer (sear and/or disconnector notch), trigger searing portion, and as a last resort trigger or hammer pin holes being worn. Obvoisly the last two would suck becuase it would mean replacing the receiver.

One of the things I would check would be the ammo, weak primers, primers set too shallow.

No matter the cause, you should get it figured out ASAP. Run-away guns can be very dangerous. You never know when it may dump a full mag and if the shooter isn't ready for it, rounds can end up all over the place.

Good luck, let us know
 
I'd head to the gunsmith for him to check out the sear, to my knowlage and what I have seen around here guns going off in bursts or full auto tend to be traced to that.
 
Sounds like its moving faster than the parts can handle it. Stronger, stiffer springs (heavier buffer & buffer spring and check disconnector spring tension).
 
This is a brand new receiver, new fire control parts, etc. Less than 400 rounds through it. I replaced all of the trigger group parts from the NM one to the regular one. I am really wondering if it isn't just the buffer/spring.
 
Had to go to the book on this one. Not knowing which parts of the trigger assembly you swapped out, the suspect parts are your disconnector, hammer (sear and/or disconnector notch), trigger searing portion, and as a last resort trigger or hammer pin holes being worn. Obvoisly the last two would suck becuase it would mean replacing the receiver.
Click to expand...

Thankfully, worn holes don't mean discarding a receiver. To the contrary, for any decent gunsmith it should be a really simple job to bore out and bush the holes: this even has the potential to true things up if done right, as well as provide a steel bearing surface to avoid wear issues in the future.

Just make sure its done by someone that knows what they're doing: drilling or reaming are NOT the correct solution! End mill or boring bar followed by reamer is the only way to go.
 
There is a possibility...

that your loads aren't strong enough.

9mm for handguns and 9mm for SMG's are two different things. Using ammunition that is too weak can push the hammer back enough to cycle and pick up another round, but not far enough to engage the disconnector.

Try this before you get into extensive part replacement:

Either buy some +P loads, or select some +P loads from a reputable manual. You'll want to use normal (115 grain) to heavy (147 grain) bullets with a good crimp. For instance, the Alliant manual lists 6.7 of Power Pistol under a 115 gr. FMJ bullet. Power Pistol is a very high energy powder; I would start at 6.1 grains, and work up from there. Minimum OAL is noted at 1.12, so don't seat below that.

At the range, for testing first perform a function check. Clear the weapon, pull the trigger and hold it to the rear. Pull the charging handle and release it, letting the bolt slam home; the hammer should not fall. Release the trigger; you should hear and feel a distinct click as the disconnector releases the hammer to the primary sear notch. Pull the trigger again, and the hammer should fall.

If the function check is normal, make sure that the bolt carrier group is lubricated properly to reduce dragging. Now, load ONE round, and one round only without the magazine. Ensure that your weapon is fully under control, and pointed at the target or backstop before you release the bolt. With the round chambered, pull the trigger and hold it. The hammer should lock back with the disconnector and not fall.

If this is satisfactory, then load two rounds in the magazine, and fire them singly.

Hope this helps!
 
Well I just cleaned it and tried the fuction test that Powderman prescribed. Everything worked just fine. All parts appear to be clean, well-lubed (where needed), nothing broken, etc. I am letting the bolt soak in some cleaning solution and then will lube it lightly. I am still almost convinced that it is a problem with the buffer being too light. A person on subguns had a similar problem, and he said:

"I have an oly in 9mm that started the same thing.
It went from 2-3 round burst to all out F/A after
a few mags. Only did it with a C.A.R. stock. Here's
how I fixed it. I cut a brass buffer that weighs 8ozs.
That stopped it. The impulse with the C.A.R. buffer was too
sharp and dropped the hammer on closing. The heavy buffer slowed down
the impulse and now you cant make it double."

I have a 6.4 ounce 9mm buffer (RRA) on order now and will see if that slows the impulse down enough to make a difference.
 
I am really wondering if it isn't just the buffer/spring.
Click to expand...
I had the same problem with my issued M16 in Basic
It turn out to be a cracked buffer which was a recurring problem with that particular rifle.
 
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