My rifle is doubling - why?

[Pro2AInPA]

At our monthly 3 gun match today, my AR decided to start going select fire on me. It happened twice today. Once it doubled, and I'm pretty sure once it was a 3 round burst.

I have thousands of rounds on this lower and this has NEVER happened before. A professional gunsmith was at the match today and he disassembled the lower and didn't see anything wrong with the FCG. I had the issue on the first two stages of the day, and for the last two stages I put my upper on another shooter's lower and had no problems, so I suppose I've isolated this to a problem with the lower?

Any ideas what causes this?

Here's a video of one of the instances:

[kurtm]

What carrier and what primer? Any change in either recently? Did you recover the "doubled" brass?, and no I didn't watch your movie, so I don't know what gear you run.

[Pro2AInPA]

It's a complete LaRue upper. CCI small rifle primers.

Like I said though, the problem went away when I put my upper on a different lower.

[MikeFoley]

what trigger?

[GunCat]

Doubling is often a result of a malfunctioning disconnector in the FCG.

With your lower only (remove the upper assembly) perform this Disconnector Function Check:

(Be prepared to stop the hammer's fall with a block of wood , your thumb, or some means of keeping the hammer from slamming into the lower receiver).

Keep the trigger pulled back and cock the hammer. As you release the trigger the hammer should NOT fall. As the trigger moves forward the Disconnector should release and the hammer get caught on the trigger. This all happens as the trigger is “almost” fully forward.

If the hammer WILL completely fall as you release the trigger (1) Check the disconnetor spring to make sure it is not kinked, bound or otherwise blocked. (2) Ensure the disconnector can freely move in its slot in the trigger (3) A worn or damaged tip on the disconnector can let the hammer release too soon and cause the hammer to fall; repair or replace the disconnector.

[mlmiller1]

Doubling is often a result of a malfunctioning disconnector in the FCG.

With your lower only (remove the upper assembly) perform this Disconnector Function Check:

(Be prepared to stop the hammer's fall with a block of wood , your thumb, or some means of keeping the hammer from slamming into the lower receiver).

Keep the trigger pulled back and cock the hammer. As you release the trigger the hammer should NOT fall. As the trigger moves forward the Disconnector should release and the hammer get caught on the trigger. This all happens as the trigger is “almost” fully forward.

If the hammer WILL completely fall as you release the trigger (1) Check the disconnetor spring to make sure it is not kinked, bound or otherwise blocked. (2) Ensure the disconnector can freely move in its slot in the trigger (3) A worn or damaged tip on the disconnector can let the hammer release too soon and cause the hammer to fall; repair or replace the disconnector.

Guncat,

That is the best set of instructions dealing with a gun problem I've seen on these forums. I don't know if what you said is correct (I'm betting it is) as I'm just learning about AR internals but your instructions were clear, to the point, & complete. Thank you very much! We need a sticky on doubling ARs with your post on it.

MLM

[Pro2AInPA]

what trigger?

Standard "mil-spec" RRA. This lower/LPK has 3-4k rounds on it.

[Stlhead]

If the ammo was reloaded, that could also be the problem. My rifle will double (or more) if the primers are not fully seated. If you reloaded military brass without fully dealing with the crimp you may have had some primers high enough to get some slam fire action. Or not, something to check.

[Laughing_jackal]

If the ammo was reloaded, that could also be the problem. My rifle will double (or more) if the primers are not fully seated. If you reloaded military brass without fully dealing with the crimp you may have had some primers high enough to get some slam fire action. Or not, something to check.

I can contest to this. I believe this is the problem I've been having as of late since reloading for .223. Kind of crazy stuff.

[esskay]

Have a buddy who had a very light trigger in his AR and occasionally when running and gunning (e.g. close targets on hoser stage) it would double on him. He suspected that it was because he was a bit loose with the gun at those times and it was essentially bump firing on him. Has anyone experienced this sort of thing?

[caspian guy]

Have a buddy who had a very light trigger in his AR and occasionally when running and gunning (e.g. close targets on hoser stage) it would double on him. He suspected that it was because he was a bit loose with the gun at those times and it was essentially bump firing on him. Has anyone experienced this sort of thing?

If you have a light trigger with minimal over travel this can easily happen.

[Middle Man]

Have a buddy who had a very light trigger in his AR and occasionally when running and gunning (e.g. close targets on hoser stage) it would double on him. He suspected that it was because he was a bit loose with the gun at those times and it was essentially bump firing on him. Has anyone experienced this sort of thing?

If you have a light trigger with minimal over travel this can easily happen.

After ruling out a disconnector malfunction and the possibility of slam fires from high primers...it's surprisingly easy to bump fire if you don't have your rifle mounted solidly into your shoulder. I've seen bump fire demos from manufacturer personnel with a heavy, clunky AK trigger and on a gritty Mac-10 clone trigger. It's easier than the ninja reload to master.

[Graham Smith]

If the ammo was reloaded, that could also be the problem. My rifle will double (or more) if the primers are not fully seated. If you reloaded military brass without fully dealing with the crimp you may have had some primers high enough to get some slam fire action. Or not, something to check.

Here is an interesting test. At the range, with the rifle pointed in a safe direction, lock back the BCG. Put a mag with a single round in it and hit the release and let the BCG slam forward. Without firing the round in the chamber, pull back the charging handle and lock back the BCG. Pickup the round that was just ejected and look at the primer. There will probably be a slight indentation in it from the floating firing pin striking it when the bolt slams home.

This is perfectly normal but if you have poorly seated primers, the anvil can be high enough to cause the round to fire when you do this. And remember that this same action takes place each time a new round is fed into the gun.

P.S. This is one very good reason why you should never practice load live ammo at home.

Edited July 19, 2010 by Graham Smith

[jeffsoward]

I may have missed it, but is the lower an RRA or just the LPK?

The reason I ask is that I had a Colt match HBAR lower and it uses a larger diameter hammer/trigger pin than a "normal" lower. When i first got mine, it did the same thing. It worried me to death. I found out that it had an RRA LPK in it. The standard pin diameter is .154" but alot of Colt's use .170". this tiny variance is enough to cause this sort of malf.

If you follow GunCat's instruction and don't find an issue, try to mic the pin holes for your lower. also mic the pins and check for any abnormal wear.