AR-15 doubles

I had that happen with my first AR that I built back in the early 1990’s. Mine was caused by out of round gas key where where the gas tube slides into the key when the bolt is cycled. I’ve read it can be caused by an out of spec disconnect, it’s spring and even poor shooter control.
 
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My S&W M&P 15 PC did that. I sent it back to the factory. They replaced the bolt. It has not doubled since.

I don't know how many issues in an AR could cause it to double, but the BCG is a possibility?
 
I would guess it's slam firing. AR15s have floating firing pins, they can slam fire if the ammo has soft primers.
 
What trigger do you have ? If it’s a light pull double action trigger . I’d say it was poor trigger finger control . When I first put in my Geisle high speed NM trigger I got a few doubles that were me not pulling through and holding .

If mil-spec trigger I’d say your disconnecter is worn out . Had a bad out of spec disconnect on a brand new Del-Ton complete rifle give doubles . Sent it back and they return it with a note saying the disconnect was out of spec , been gtg ever since .
 
It never went "full auto" but just double tapped about 4 or 5 times. Any ideas as to the cause and cure?
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There are a number of things that can result in doubling,

One of them is the ammo, high primers or too soft primers can result in a slam fire. However, bad ammo tends to be in bunches and the likelihood of a soft primer or high primer being the second round, and only the second round "4 or 5 times" is rather low. I would not put "bad ammo" at the top of my list, based on your description.

Inadvertent "bump firing" is possible.

Some issue with the engagement between the hammer, disconnector and trigger are the more likely causes. This can be due to incorrect tolerances, wear, and/or something changing the normal movement arc of the parts.

First thing to check is easy and external, check the hammer and trigger pins to see if either has "walked out" Incorrect assembly of the lower group can allow them to do that, and that can change things enough to allow the gun to double, or even run away continuous full auto fire. Pins flush? move on..Pin sticking out? find out why and correct it.

engagement surfaces.. hammer and disconnector, hammer and trigger. Many AR lower parts were made from "sintered metal". They have a hardened surface at the contact points but below that the metal is rather soft. IF that original hard surface is worn through (or polished through by some amateur "trigger job) wear of the softer metal is rapid, and when things wear enough, malfunction will result.

Something causing the disconnector to "stick" and not catch the hammer is possible. If the hammer follows the bolt carrier down it USUALLY does not fire the round. But, it might, and that MIGHT account for doubling. And it could be an intermittent problem. Not high on my list but not impossible.

One important detail needed to help diagnose the problem from a distance, did you fire the round and the next round fed in fired immediately?? Or did it go off when you released the trigger prior to the next trigger pull??

Cycle the gun empty repeatedly with the trigger held back, then released if at any time the hammer falls, its a gun, not an ammo or shooter issue.

The fix is to replace the bad parts. Knowing which are the bad parts, and why, is the trick.

Hope this helps,
 
Had some of that with an Armalite AR-10 T, Factory unaltered trigger.
It had about 4000 rounds on it.

It started only when I fired it.I was doing a gradual ,slow trigger squeeze.
It was not happening with a more aggressive trigger pull Pretty much a two round burst.

In any case.a new Gisselle cured it.

Of the triggers I've used,the "bang for the buck" nice trigger is the Rock River National Match 2 stage
 
Best bang for my buck is the Larue trigger. Slightly lighter pull then the RR and much quicker reset . I’ve found my RR triggers seem to be slow on rest , almost as if my finger can release the trigger faster then it will move forward for reset .
 
A relative of mine bought a "Franken Gun" AR-15 several years ago that would occasionally double. At first he thought that perhaps it was slam-firing so he thoroughly cleaned the bolt and bolt carrier and replaced the firing pin with a titanium one. The doubling continued and was eventually solved by replacing the FCG with a drop-in modular one from Chip McCormick. After replacing the FCG, the rifle functioned correctly until it was eventually sold a few years later.
 
"...It never went "full auto"..." Like AMP says, doubling is FA.
You shooting off a bench? The ammo light reloads?(knew a guy who had a semi'd Uzi that'd rock and roll with cast bullet reloads) Your "what I could find" trigger especially light?
If the Bushmaster doesn't do it with the same ammo that eliminates the ammo as the cause.
Slam fires are about improperly loaded ammo, not the gun. ALL primer cups are the same too. There's no such thing as a soft primer.
 
ALL primer cups are the same too.
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No, they're not. Same in size but beyond that some are different.

There's no such thing as a soft primer.
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yes, there is, in comparison to other primers. Soft, and hard primers are terms used describing primer sensitivity to impact. The common assumption is that soft and hard refers to the primer cup metal, and in certain examples, that may be the case, but it also covers primers going off reliably from a "soft" or "hard" firing pin strike.

People have "tuned" guns (DA revolvers, primarily) for better trigger pulls which resulted in lighter (softer) than standard hammer strikes, and found that often only one brand of primer (Federal IIRC) will reliably fire. Other brands aren't reliable in their gun so the shoot only the one that is, and refer to the others as "harder" primers. Meaning harder to reliably ignite.

CCI sells a "mil-spec" rifle primer, that is different than their regular primer. In this case there is a difference in the cup metal, (there maybe other differences as well, but i can SEE the difference in the cup metal)

Chamber a GI round (or GI spec primer reload) in my M1A then take a look at it. The floating firing pin bounces off both the same but the GI spec primer is generally unmarked while the civilian grade one has a small dimple where the pin hit it.

NONE of my reloads with CCI 200s has ever gone off (slamfire) from this, but there is always a visible mark, so the cup metal is different, and I'd say its a bit "softer" than the GI stuff.
 
You could always swap the lowers to see if the problem follows. It an easy way to test if you don't see anything obvious to start with?
 
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