Topic: Slam-fire

[9Shooter]

I was out shooting yesterday with friends and had an unusual problem.  I think I understand what was happening but figured I'd ask the more experienced.

I brought a number of rifles and only after I got there did I realize I had left my AR mags at home.  My wife has been wanting to shoot the AR for a while so rather than wait until next time, I had her drop in a single round, close the bolt, and let it rip.

That method worked fine the first few times, but then when releasing the bolt on the 3rd or 4th round, it fired.  Out of 15 rounds or so it happened 3 times. My guess is that with a round in the chamber already, rather than being carried in by the bolt, the firing pin hit the primer with enough force from forward momentum when the bolt closed that it set off the primer.  Is this a reasonable explanation?

When I got home I stripped it down for cleaning and did not notice anything unusual with any of the parts.  I'm using purchased reloads (in case that's a factor), Stag upper, and have never had this problem before when using magazines.

[No-One]

My guess is that with a round in the chamber already, rather than being carried in by the bolt, the firing pin hit the primer with enough force from forward momentum when the bolt closed that it set off the primer.  Is this a reasonable explanation?

The AR-15 has a floating firing pin so your deduction is completely reasonable . Some people have actually had the same problem when using softer primers which has led some manufacturers to recommend the use of Magnum primers in AR loads .

[ballardw]

I'd say you are right on for what happened.

 if you find youself in a similar situation try riding the bolt with the carging handle to keep the speed down and then use the forward assist to ensure the round is seated.

[9Shooter]

Thanks for the replies, that confirms what I was thinking.  Only after I got home did I  think about riding the bolt forward instead of letting it go from the locked position.  Next time I think I'll just remember to pack the mags

[SEVERED]

This is one reason CCI makes #41 primers too, they are harder to prevent this from happening, from what I have been told.

[danno12345]

Slam fires suck.   

My dad's Luger will do it if you put one in the chamber & drop the "slide".  Scared the crap out of us the first time it happened to Dad.  The round went into the dirt about a foot & a half from my shoe.  I looked at him like WTF!?  He swore up & down his finger wasn't on the trigger.  I believed him & have since confirmed that it will do it all by itself. 

It's a useful piece of information, and one that I share with anyone that wants to try the gun.

Dan

[Nealio]

Usually you have a higher change of this "slam fire" happening if you drop a single round in the chamber because you are pointing the gun down towards the ground when you release the bolt.  You now have gravity helping you out (both in speeding up the bolt and putting force on the firing pin), as well as reduced drag on the surfaces the bolt rides on.
I've inspected my primers after doing this and they always have a mark on them.  This is why guys switch to lightweight titanium firing pins.

[9Shooter]

Interesting you mention a mark on the primer.  On some of the brass I picked up afterward, there were some with what looked like a small doughnut shape in the center instead of the usual indentation.  I chalked that up to there being no hammer behind the firing pin.  That would allow the primer to force the pin back into the bolt when it goes off, leaving a small indentation in the center where the pin struck and ignited it and a small ring around that where the primer expanded back into the pin hole.

[tgibson]

I just got done with a practice session and took a careful look at every round I took out of the gun when I was unloading and every single one of them had a mark on the primer.  I guess it is all in what you use.
Adios,
TG