Had a Slam Fire

[MickB]

I assembled a new rifle for a buddy.

Aero upper and lower, Spike's mid-length gas 16" barrel with pinned gas block and 5.56 chamber, AIM M16 (full mass) BCG, RRA lower parts kit, DSG mil-spec carbine buffer tube with 3oz buffer, JP trigger with bobbed GI hammer.

Checked headspace with a GI gauge - passed, no problems.

Dropped a round with no primer in the chamber, bolt closed with no issues.

Dropped a live round in the chamber, turned rifle barrel up and it wouldn't fall out. Tried to pry it out with a finger. Still wouldn't come out.

Figured I would drop the bolt on the round and pull it out with the extractor (that was one mistake).

Hammer was back, safety was on, finger off the trigger and rifle was pointed in safe direction.

I hit the bolt release and BOOM!

I was stunned for a good while, not expecting the round to go off. Small chunk of ground missing in front of me.

No injuries.

Round in question was a WCC (Israeli manufacture), 55gr Hornady FMJ behind 24.8gr AA2230 with a CCI400 small rifle primer.

Spent case has a raised area around the firing pin indentation

After regaining my composure, I tried to figure out what had happened.

Pulled everything apart and with BCG removed, I dropped another round of the same batch into the chamber. It didn't fall in with a "clink", so I pushed it out with a rod.

Ran a patch through the barrel, and rounds now dropped in with a "clink" and fell out freely.

My other mistake was never running a patch down the bore after assembly.

I will be doing further live round testing over the next few days, but my theory at this point is that the chamber, neck area was dirty. With the round not fully in the chamber, dropping the bolt resulted in the slam fire due to firing pin inertia.

Any other theories?

Firing pin too long, perhaps?

Any possibility of hammer follow through?

As I said, i will be doing further testing over the next few days.

Mick

Edited July 29, 2014 by MickB

[Steve RA]

I'd go with the dirty theory unless it does it again.

[Shooter115]

Are you sure the trigger is functioning properly and the hammer didn't follow the bolt down?

[ccoker]

Are you sure the trigger is functioning properly and the hammer didn't follow the bolt down?

When I was reading the build this caught my attention "JP trigger with bobbed GI hammer."

[MickB]

This thought has occurred to me, and will be testing for it after work.

I have installed bobbed GI hammers with JP triggers a number of times, all without issue. That combo and bobbing procedure is even there in the JP instructions.

I will say that the bobbed hammers I have used in the past were all rounded DPMS. This one was a notched RRA hammer.

Mick

Are you sure the trigger is functioning properly and the hammer didn't follow the bolt down?

When I was reading the build this caught my attention "JP trigger with bobbed GI hammer."

[Shooter115]

Are you sure the trigger is functioning properly and the hammer didn't follow the bolt down?

When I was reading the build this caught my attention "JP trigger with bobbed GI hammer."

Which in itself is fine......... if the trigger and disconnector are adjusted properly. With the JP trigger the disconnector needs to be hand fit. It doesn't too many extra swipes on the stone to have an unsafe trigger.

That said a properly set up JP trigger is still the best you can get.

[kurtm]

The raised primer is the clue here guys. It was indeed a slam fire! I've had 3 so far, and seen several others! It scares the bejesus out of you. I now run Titanium firing pins in all my ARs and haven't had one since!. It is just part of the AR system. Make sure you don't have a large collar firing pin, they weigh the most and in one of my rifles with a low mass carrier it would slam fire frequently while shooting and twice upon closing the bolt!

On a side note I ALWAYS cringe when folks get the load and make ready with their ARs and they jam in a magazine and drop the bolt with the rifle pointing up at 45 degrees. I ALWAYS point it at the berm, and make a note to tell the R.O. that the safety is on and my finger is straight before I drop it. Remember a slam fire doesn't make you go home but a round leaving the berm does.....and where does it go?

[pmt]

On a side note I ALWAYS cringe when folks get the load and make ready with their ARs and they jam in a magazine and drop the bolt with the rifle pointing up at 45 degrees. I ALWAYS point it at the berm, and make a note to tell the R.O. that the safety is on and my finger is straight before I drop it. Remember a slam fire doesn't make you go home but a round leaving the berm does.....and where does it go?

Thank you for that reminder.

Edited July 28, 2014 by pmt

[MickB]

Just retested the trigger on the rifle. I could not recreate any hammer follow through, and the trigger passed all safety function checks.

I disassembled the BCG. Cotter pin was tough to remove. Yep, it is bent.

At first the firing pin wouldn't drop out. A second later, it falls out into my hand, along with the sound of a small "clink" of a VERY small metallic piece of debris.

It is possible that this small piece of debris, along with the new, uncleaned chamber, is what caused the slam fire.

I also measured the firing pin length and this pin is LONGER than the two spares sitting in my parts box and the Colt firing pin pulled out of my Tac Limited rifle.

The firing pin in question has 0.005" more protrusion than the others.

Mick

Edited July 29, 2014 by MickB

[MickB]

I measured the slam fired case in my RCBS micrometer gauge. Dead nuts on the zero line, showing as perfect headspace...

Mick

[kurtm]

MickB, NOTHING is wrong with your rifle!

[ccoker]

On a side note I ALWAYS cringe when folks get the load and make ready with their ARs and they jam in a magazine and drop the bolt with the rifle pointing up at 45 degrees. I ALWAYS point it at the berm, and make a note to tell the R.O. that the safety is on and my finger is straight before I drop it. Remember a slam fire doesn't make you go home but a round leaving the berm does.....and where does it go?

Thank you for that reminder.

Definitely reinforces "muzzle in a safe direction" (and 45 into the air is NOT)

[Shooter115]

MickB, NOTHING is wrong with your rifle!

So is the "slam fire" typically just the by product of a lightened bolt carrier and increased bolt velocity. I've never had this happen in a dozen or so rifles and I have no idea how many thousands rounds, but I've never used anything but full weight carriers. I load all my ammo with standard CCI SR primers. If this were even a remotely common occurrence in the 55 years the weapon has been in service, one would think there would have been a design change along the way. Not saying it can't or hasn't happened, because obviously it has happened to you, but has it ever happened with a standard weight carrier where there weren't other potential issues with the rifle.

[kurtm]

Yes it has happened with standard weight carriers. I also have shot a couple of thousands of rounds ( I have burned out 14 AR-15 barrels over the years, and have multiple rifle as well) and it has only happened 3 times to me, but if you attend Camp Perry you will usually see a couple a year in service rifle,

[MickB]

The slam fire I had with this build was with a full weight carrier and standard CCI 400 SR primers. First slam fire I have ever encountered.

My models at this point are:

Debris in firing pin channel (found a small piece of metal shard).

Firing fin protrusion too long.

Overly sensitive primer.

I'm moving away from the dirty chamber model at this point.

Further testing later today. I have replaced the firing pin with the shortest one in my parts box and we will see what happens...

Mick

[MickB]

Testing the rifle today with the shorter firing pin and different ammunition types gives it a clean bill of health.

All ammo had very light primer marks from dropping the bolt. By light, I mean light, as in very normal for an AR pattern rifle.

Ammo tested was IMI M193 military (hardest primer), PMC bronze .223, and the same batch of my reloads using CCI400 SRP.

I am going to conclude that the slam fire occurred either due to the debris in the firing pin channel, or the firing pin is slightly too long.

I will take the suspect firing pin to Accuracy Speaks and see if Derrick can measure it using a proper gauge.

Lessons learned. Always fully clean new parts, and when doing live testing, keep the muzzle pointed in a SAFE DIRECTION!!!!

Mick

[StealthyBlagga]

It sounds like you have a root cause identified. Just for interest, can you post a photo of the primer of the slam-fired case, maybe compared with a regular fired case? It would be good to see the difference in indentation depth.

[MickB]

Here is a picture of the fired cases.

Slam fired case is on the right.

Case on the left is with new, shorter firing pin installed.

The slam fired case looks like it was punctured.

Mick

Edited July 30, 2014 by MickB

[mpeltier]

Figured I would drop the bolt on the round and pull it out with the extractor

I hit the bolt release and BOOM!

Mick

This may be (in addition to the long firing pin and soft std rifle primers) the full recipe for your slam fire. By having the cartridge in the chamber already (as opposed to the bolt carrier stripping it from the magazine) your bolt had a running start to the chamber, greatly increasing your chances of a slam fire.

[kurtm]

Darn fine point Mark!

[dauntedfuture]

Looks like the case on the right was loaded a l little hot. I have had the same thing depending on the primers. The only slam fire I ever had was with rem 6.5's, I never used them again. This was a great thread to read. When shooting NRA HP you simply drop the round in the chamber and slam the bolt; I don't know of any issued with a proper rifle or ammo to date. On the M1A/M14, you needed to load the round into the magazine then release the bolt because of the floating firing pin.

[cas]

Yeah... Don't do that.

Putting a round in the chamber and dropping the bolt on it I mean. That has been known to not only make some rifles slam fire, but also disassemble themselves in the process. (M1/M14/M1A firing slightly OOB)

[MarkCO]

Figured I would drop the bolt on the round and pull it out with the extractor

I hit the bolt release and BOOM!

Mick

This may be (in addition to the long firing pin and soft std rifle primers) the full recipe for your slam fire. By having the cartridge in the chamber already (as opposed to the bolt carrier stripping it from the magazine) your bolt had a running start to the chamber, greatly increasing your chances of a slam fire.

i had some uppers to test this morning...

Put a standard firing pin in a JP low mass carrier with a JP XP buffer spring, dropped the bolt with the release on a full mag...all good. Locked it in as tight as I could against the rail on the bench and out of 5 trigger pulls, got 2 slam fires. Then, locked down and pointed into the berm, I manually placed a round in the chamber, no magazine, and dropped the bolt...nothing the first time. I tried it 10 times and I got slam fires twice. Went back to the Ti firing pin and the standard buffer spring...I tried for 20 minutes to get a slam fire and I could not. Primers were WSRs by the way.

[MickB]

Interesting you point that out. For the M14/M1A, there are very clear instructions in the box not to put a round in the chamber and drop the bolt due to potential slam fires.

I have not seen such a warning for the AR15.

However, as has been pointed out above, dropping the bolt on a chambered round and no mag only increases the bolt velocity and serves to increase the chances of a slam fire.

Mick

Looks like the case on the right was loaded a l little hot. I have had the same thing depending on the primers. The only slam fire I ever had was with rem 6.5's, I never used them again. This was a great thread to read. When shooting NRA HP you simply drop the round in the chamber and slam the bolt; I don't know of any issued with a proper rifle or ammo to date. On the M1A/M14, you needed to load the round into the magazine then release the bolt because of the floating firing pin.

Edited July 30, 2014 by MickB

[MickB]

Mark,

interesting test results. makes a great case for the use of a Ti firing pin in the LMOS carrier.

I believe my case was either due to a stuck firing pin, or too long a firing pin, or a combination of the two.

I am curious as to what would be the result of the standard firing pin in the JP LMOS carrier and STANDARD buffer spring.

Mick

Figured I would drop the bolt on the round and pull it out with the extractor

I hit the bolt release and BOOM!


Mick

This may be (in addition to the long firing pin and soft std rifle primers) the full recipe for your slam fire. By having the cartridge in the chamber already (as opposed to the bolt carrier stripping it from the magazine) your bolt had a running start to the chamber, greatly increasing your chances of a slam fire.

i had some uppers to test this morning...

Put a standard firing pin in a JP low mass carrier with a JP XP buffer spring, dropped the bolt with the release on a full mag...all good. Locked it in as tight as I could against the rail on the bench and out of 5 trigger pulls, got 2 slam fires. Then, locked down and pointed into the berm, I manually placed a round in the chamber, no magazine, and dropped the bolt...nothing the first time. I tried it 10 times and I got slam fires twice. Went back to the Ti firing pin and the standard buffer spring...I tried for 20 minutes to get a slam fire and I could not. Primers were WSRs by the way.