Cause of slam-fire in my 6.5CM AR10?

[mel]

whats the main reason for an ar to slam fire , ive never ever had one happen till today, happened on the first round out of the mag when i dropped the bolt

[Klem]

The momentum of a free floating firing pin hitting a primer sensitive enough to go off.

This is why many of us prefer thicker cupped primers, like CCI450

What is the primer or factory ammo?

[HuntTXhogs]

Shouldn’t happen with factory ammo and factory built guns. Anything specific about your rifle or ammo ?

Slam fires occur when the bolt goes into battery and the floating firing pin strikes the primer with enough force to set off the round without the operator engaging the trigger.

You can more often cause a slam fire by rechambering a particular round over and over again - the first round from the mag was it chambered many times before the event ?

Or If you drop the bolt onto a round that you’ve hand fed into the chamber with the magazine out of the gun.

[mel]

this was actually in my ar-10 cm handloads with a cci #200 primer , first time loading the round inserted the mag , had the gun down on the back of the flatbed on my bipod getting ready to shoot some 5 round groups , just dropped the bolt with the release like normal and it fired, while my cm and grendel are both fairly new to me my 5.56 ar has a few thousand rounds threw and ive never had it happen with it i load for that with cci 400's

[OneHitWonder]

Get out the q-tips and clean the firing pin channel. The pin should have bounced back after the bolt closed. Something created resistance. The weight of the pin should not create the force to fire without the hammer spring power.

As a note: drop the bolt on a round. Eject that round and you notice a small dimple where the pin bounces off the primer.



Question: Are you using a heavy buffer or extra power recoil spring?

[mel]

Get out the q-tips and clean the firing pin channel. The pin should have bounced back after the bolt closed. Something created resistance. The weight of the pin should not create the force to fire without the hammer spring power.

As a note: drop the bolt on a round. Eject that round and you notice a small dimple where the pin bounces off the primer.



Question: Are you using a heavy buffer or extra power recoil spring?

i had just installed a strike industries flat wire buffer spring in it about a week ago , and also just upgraded the bolt to a brownels nickle boron bcg , prolly 20-30rounds on each , i did notice the firing pin on this new bolt does seem to protrude more then the bolt i took out when u push it forward

[A5BLASTER]

this was actually in my ar-10 cm handloads with a cci #200 primer , first time loading the round inserted the mag , had the gun down on the back of the flatbed on my bipod getting ready to shoot some 5 round groups , just dropped the bolt with the release like normal and it fired, while my cm and grendel are both fairly new to me my 5.56 ar has a few thousand rounds threw and ive never had it happen with it i load for that with cci 400's

Sounds like the primer you used is too soft for the AR platform.

[Rosecrans1]

Maybe multiple issues coupled with what Klem and Blaster wrote. I won't use CCI 200's in an AR.

[tdbru]

Mel,
i'm sure you know all this but here goes:
1. primers have to be seated all the way into the bottom of the primer pocket. do you clean pockets? feel the primers on the rest of the CM ammo. any high primers? are all the rest below flush?
2. you could try CCI34 primers i suppose. but i don't think primer cup thickness varies as much in LR as it does on SR primers. i could be wrong.
3. try a different brand of primer
4. measure the length of the firing pin. is it longer from the stop flange to the tip than is should be? compare this length to the original firing pin length from the original BCG.
5. is the tunnel that the firing pin slides back and forth in, from end to end, gummed up in any way that might have held the FP firmly forward during chambering? i think this was mentioned. is there grit in the firing pin hole in the bolt head that might have wedged the firing pin forward?
6. is the bolt shorter from the bolt face to the end of the tail that the firing pin stop flange hits? this would allow the firing pin to stick out further too. probably have the same affect as a longer firing pin. compare the measurement to your original bolt.
7. did the mfgr forget any firing pin machining steps that might have left more mass on the firing pin than "normal"? does the firing pin look like it has "normal' contours? does it look like the original?
8. did the buffer spring strength go up such that the bolt velocity going into battery is quite a bit higher than before? you said you changed buffer springs. does the new one have a much higher spring constant?
9. is the new BCG significantly lighter in weight than the one you had earlier? that would increase the bolt velocity going into battery too.

so it could be a little bit of a lot of variables.
-tdbru

[tdbru]

Mel,
one other thing to check. firing pin tip protrusion. i'm sure there's a spec somewhere. it might be just a stack up of tolerances, bolt length from the face to the tail being on the short side, firing pin length from the stop flange to the tip being on the long side. combination will give too much firing pin protrusion length. the mfgr should have a spec. see if that's it.
-tdbru

[FRB6.5]

Another thing to note, AR-10's really are not standard across all brands, everything is not interchangeable. Some AR-10's have a spring over the firing pin to compensate for the increased weight of the pin.

[mel]

thanks yall , apparently i have alot of stuff i need to look at lol

[lazyengineer]

What style bolt release do you have? Factory, or one of the extended ones that are closer to the trigger?

[FLshooter]

Around 10 years ago,my Colt 6920 slam fired on me while I was Benchrest shooting. All I had was five rounds in the 20 round magazine and it shot them all! Good thing I was holding onto the rifle tightly.I was shooting cheap military bulk ammo I got for cheap at a gun show.
I took the rifle apart from no problems. Checked the hammer and trigger springs no problem. Also had my buddy works at a tactical shop look at it he couldn’t find anything wrong. Went back to the range a few days later with different ammo and it shot flawlessly . Never had a problem with it since.We both figured it had to be the ammo.

[montana]

If the disconnect, "on a mil spec trigger does not properly fit" slam and double burst firing is inevitable. This was quite common on the older Olympic arms rifles. I filed the bottom of the disconnect leg to get proper purchase to fix the problem. Surprisingly, most AR owners know very little about this today, with the advent of better quality control in manufacturing.