ARs have a free floating firing pin that when the bolt closes without having to strip a round off the magazine could close with enough force to set a soft primer off. One theory.

On the other hand, there are youtube videos of guys putting the same round back in the gun and closing the bolt on it one hundred times or more with no discharge. The dimple in the primer just kept getting bigger and bigger. I can't remember if the guy was using a magazine in the one I watched or not.

 

How dirty is the gun? Any chance the firing pin could be jammed forward from crud in the BCG? I had an old 22lr that happened with, it basically turned into an open bolt full auto.

 

Its called a Slam Fire.

While VERY rare, it can happen. AR15s have a floating firing pin, meaning there is no spring or anything holding that firing pin to the rear. As the bolt slams home, momentum carries that firing pin forward and it strikes the primer. To show this, if you pay attention to just about every round that gets chambered in your AR, even if you don't fire it, you will probably see a small mark on the primer where the firing pin contacted it.

So, one issue that can make it more susceptible to a slam fire, is a soft primer cup. Those of us who reload, especially for ARs, know that some primer brands have a softer cup than others, federal being one of the softest. Also, there are primers made specifically for ARs that have a harder cup, making it harder for the primer to go off.

I too have a Rock River operator. I reload, and shoot factory rounds. In THOUSANDS of rounds, I've never had a slam fire. However, I can see the indent from my firing pin on just about every round that I chamber. Just a little reminder to always keep her pointed in a safe direction when loading up.

Also, to help keep it from happening again, always load your magazine, then let the bolt strip that round out of the magazine and chamber it. Helps to slow it down just a tad.

Edit: I say that with the assumption there are no other issues with your rifle, and the trigger group is not malfunctioning.... which is highly doubtful. I'd put money that it was a slam fire.

 

Its called a Slam Fire.

While VERY rare, it can happen. AR15s have a floating firing pin, meaning there is no spring or anything holding that firing pin to the rear. As the bolt slams home, momentum carries that firing pin forward and it strikes the primer. To show this, if you pay attention to just about every round that gets chambered in your AR, even if you don't fire it, you will probably see a small mark on the primer where the firing pin contacted it.

So, one issue that can make it more susceptible to a slam fire, is a soft primer cup. Those of us who reload, especially for ARs, know that some primer brands have a softer cup than others, federal being one of the softest. Also, there are primers made specifically for ARs that have a harder cup, making it harder for the primer to go off.

I too have a Rock River operator. I reload, and shoot factory rounds. In THOUSANDS of rounds, I've never had a slam fire. However, I can see the indent from my firing pin on just about every round that I chamber. Just a little reminder to always keep her pointed in a safe direction when loading up.

Also, to help keep it from happening again, always load your magazine, then let the bolt strip that round out of the magazine and chamber it. Helps to slow it down just a tad.

Edit: I say that with the assumption there are no other issues with your rifle, and the trigger group is not malfunctioning.... which is highly doubtful. I'd put money that it was a slam fire.

Great info, thanks for sharing !

 

Also check the primer seating depth on those sub sonics. I have read that that can also cause a slam fire if they aren’t seated deep enough. As with everyone above, it hasn’t happened to me, so I’m just repeating what I heard on the internets.

 

Thank guys!

Gun isn't dirty....probably less than 100 rounds since I bought it and has been cleaned since.

I'm all about gun safety...this one was a bit eye opening for me...makes me think about when/where I will load this gun from now on.

 

CCI and several others make a specific primer for auto loaders. The primer cup is much harder to resist a slam fire....If I were a betting man, I would wager the subs your shooting were intended to be used in a bolt gun and have some sort of match primer.

Glad nobody was injured, those cardinal rules of safety sure do work!

 

Id look more towards the trigger. Rr has had issues in the past. If its a new gun make sure all the springs are in and in right, function test, if you can repeat it take note if the hammer fell or not.

An oldie but have heard of this before