AR15 slam fire

[edfardos]

I've had two slam fires in my last 100 rounds (two different outtings). Basically fires twice with a single trigger pull. The AR15 in question has fired roughly 1500 rounds total, with never a glitch.

I recently switched to CCI400 primers since I couldn't find any CCI450(magnum) primers. I've fired 300 of these, and the two slamfires were somewhere between shot 200 and 280.

The two slam fires were also with a new lot of FC brass, and I normally use PMC.

I'm attaching a picture of the two brass involved in this uncommanded double tap. The picture looks exactly like dozens I've seen on the internet. One's an "inny" and ones an "outty".

Any idea what's happening? I'm assuming sensitive primer, or possibly the brass is involved since those are the only variables. The AR15 lower function-checks just fine on the bench. My firing pin puts a pretty good ding in the primer when the bolt closes, just like others I've seen on the internet.

How unsafe is this condition? I still have 350 live rounds with these primers, and a box of 1400 unused primers.



curiously,
-edfardos

 

[Guilty]

I would tend to first suspect that your finger is hitting the trigger on recoil rather than a slam fire, especially if you have installed a trigger with a lighter pull than a heavy pull battle trigger for example.
If you still suspect a slam fire, maybe check the primer seating depth and make sure that you aren't seating the primers too high.
If you continue to have problems, change to a CCI 41 primer which has a harder cup. A little trial and error testing is in order.

 

[carbine85]

It's best to use harder primers in the AR or any semi-auto with a floating firing pin. Check the seating depth of the primers. The AR isn't known for slam firing. Assuming the trigger group is ok and the firing pin is floating I would suspect the primers.

 

[edfardos]

thanks for the feedback. I'll double check primer seating on the next trip, mark any that are possibly high or flush.

I recall researching the difference between standard and mag primers, and CCI used thicker material for their cups. My current working theory is the new, thinner primers are detonating when the firing pin hits the primer as the bolt closes over the cartridge.

The other wild card is this new lot of commercial FC brass. Is there anything brass might contribute to a slam fire? Everything is full length sized, no different from the last 1500 rounds of PMC I reloaded.

I'll switch back to CCI450/mag primers in the future, but the big question is if my existing 350 live cartridges are safe to discharge. If the firing pin can hit the primer when the bolt closes, it means the rim has already passed over the extractor and it's highly likely the bolt has locked right? What are the odds of an out-of-battery discharge?

thanks again,
-edfardos

 

[243winxb]

Always load a round from the magazine.

What are the odds of an out-of-battery discharge?

Not with an M16 type, unless maybe a broken firing pin?? Yes for the M1 & M14 .

 

[TBH]

I agree with Guilty. I have a Jewel trigger and set it for 2.5#. I have had several double taps. They would be fun if you were expecting them, but not by surprise.

I am now set at a nice crisp 4#. You need enough weight to make your trigger finger pull through. Now if you have a stock creepy, 6-7# trigger, then it may be the primers. I use only CCI 450. I am sure deep down you already know if the 400s are slam firing, you need to pull the rest of them. Don't feel alone, I loaded 400 69g SMKs and twice had powder hang up in my powder measure funnel. Dillion 550b, N140. Not sure why, but I placed an order at Midway for a puller.

Good luck!

 

[ole farmerbuck]

Can the firing pin be gummed up causing this?

 

[blarby]

I would like Slamfire1's opinion.

Maybe its a trick of the lighting on that pic, but my primers still set deeper than that after firing.


With that said- when you are having malfunctions, its best to investigate fully before you consider anything "safe".

I would have stopped after the 1st double.


Its entirely possible you are getting a recoil/trigger doubletap.... Its entirely possible you are not.

I've never had a slam using cci400's, but you do hear about them using Federal smalls enough that I wont use them in this application.



Given that we're into the cold weather months, that can't be ruled out as a failure point yet- but that would be more lubricant binding on your firing pin, unless i'm mistaken Edit- you'd have more than a lube issue. Gloves and the trigger have caused this on cold days- i've seen it ! That was a fun end of a magazine.....


I think we've hit on most of the common cause/variables. Now its time to find out which one !

#1- check the firing pin for movement/non movment due to gumming. If you play with your bolt/carrier assy when you take it out, you could rule this AND serious mechanical failure right out- see havys post below. You can simulate that on a bench.
#2- weight check your trigger pull
#3- fire some factory ammo and see if you get a duplicate

 

[MEHavey]

I wager you have doubling (probably recoil/trigger-finger bump) and/or the disconnector failing (to capture the hammer until the trigger is reset). The hammer then rides the bolt home.

Luckily, the AR15 design does not allow to firing pin to protrude from the bolt face until the lugs are pretty much locked -- hence danger from a classic Garand-type (and true) Slamfire is vanishly small.

 

[edfardos]

wow - great info guys. Just to touch on the points:

1) Trigger group function-checks perfectly on the bench (disconnector holds).
2) No lube on the firing pin other than some oily fingers putting it in.
3) It never did this in the last 2000 rounds with CCI450-mag primers
4) Trigger is a very heavy, "stock creepy" S&W trigger group
5) The extruded primer bump indicates a detonation with no hammer behind the pin?
6) I've tried to bump fire it for fun in the past... poorly.
7) First slam fire was off of bags (slow pull), second was off knee (fast pull)

Two variables are the non-mag primers and this new FC brass.

first 2000 were mag-primers with PMC brass -- no slam fires
next 100-200 were non-mag primers with PMC brass -- no slam fires
next 201-300 were non-mag primers with FC brass -- slam fires on #225 and #275

AR15 is detail stripped/cleaned every 50 rounds. I tripple checked the firing pin, good dome on tip, amazingly straight, slides with ease, no excessive grease, no flakes in the tunnel/bolt.

For posterity, the lot# of these primers is "B11U". and I have 1400 to trade, if you have a bolt-action small-rifle

I'm still leaning towards the non-mag CCI400 primers, but I really want you guys to challenge me and think of additional alternative causes. Especially if we can find a real safety issue here. I'll shoot some of my reserve stock with cci450 mag primers for now.

thanks again,
-edfardos

 

[243winxb]

Slam Fire

This photo shows that the hammer was not dropped and in contact with the firing pin. The slide closing at too high a velocity will let the inertia firing pin hit the primer. A weak ejector spring will not slow the bolt, as much as it should.

 

[DC_art]

I recall researching the difference between standard and mag primers, and CCI used thicker material for their cups. My current working theory is the new, thinner primers are detonating when the firing pin hits the primer as the bolt closes over the cartridge.

I think you missing the intended direction of using a harder primer. Using a CCI 450 (or any SR Magnum) primer would be an entirely different load. The CCI 41 should be functionally equivalent to the CCI 400 (same load) but with a harder cup.

 

[rcmodel]

first 2000 were mag-primers with PMC brass -- no slam fires
next 100-200 were non-mag primers with PMC brass -- no slam fires
next 201-300 were non-mag primers with FC brass -- slam fires on #225 and #275

Make sure you remove all the military primer crimp before seating primers.

I ran into that doubling problem years ago with my Colt when I was smashing in CCI 400's with my loading press, even though they didn't want to go.

Smashed primers are more sensitive then the same primers seated with a normal amount of force.

I have never again had a double after I started swaging primer pocket crimps, and seating with an RCBS hand priming tool so I could better feel what was going on.

rc

 

[edfardos]

Weak ejector spring? You've got me interested DC_art.

The ejector spring is the cylinder that compresses behind the rim of the cartridge right? It's all I can do to compress this with a thumbnail. And this is not to be confused with the extractor spring (with rubber o-ring). I would expect both of these springs to slow the closing of the bolt. You've given me something to think about - thanks.

rcmodel -- you've never been wrong about anything on this forum -- ever .
I give primer seating some serious muscle on my rcbs-pro-2000. To the point where I've been wondering if I'm damaging the primer. But that applies to my previous 2000 flawless rounds as well. Maybe I'm damaging the CCI400 (non-mag) primers during seating... again, something to think about, but w/o the muscle, I get proud (high) primers from time to time.

243winxb: great picture. That's exactly what I'm seeing. I think we're all agreed (agreeing) it's an inertial strike and not a hammer strike? That explains the extruded primers.

My plan is to shoot some of my old-stock cci450 mag-primers and see if I can replicate the slam fire, which will point at the firearm. Then I'll try some of the newer CCI400 non-mag primers and see if it comes back. That should be a sufficient test. At this point I think the change in brass (FC) is probably a coincidence.


more to think about, thanks,
-edfardos

 

[rcmodel]

I think the Fed brass had more remaining primer crimp left in it then the other two brands.

Smashing primers in past a crimp!
That's what does it I tell ya!!

rc

 

[edfardos]

DC_Art, I'm using H335, which calls for a magnum primer in the Speer manual (but no other manual). It's technically a ball powder. Accuracy results have been optimal with the mag primer (and just fine with the non-mag primer, except for the slam fire). I load nowhere near max (23.5grains under 55grain hornady).

--edfardos

 

[edfardos]

rcmodel - this is commercial brass - no crimp. I can't get my rcbs pro2000 to reliably prime de-crimped brass (1 in 12 goes in at an angle, no matter how much crimp i remove). I have bags of unused crimped brass in the garage :/.... I really like your theory on over cramming primers into pockets. I'll try with less force, yet make sure they're still not high.

--edfardos

 

[DC_art]

Edfardos,

I'm not the one discussing the ejector spring. I'm just cautioning you about substituting in a magnum primer. That would require a different load. That would be a whole new variable in the equation you're trying to solve.

Art

 

[rcmodel]

I can't get my rcbs pro2000 to reliably prime de-crimped brass

Then you need to up-grade to a better method of priming.

My RCBS hand priming tool has worked flawlessly for a lot of years.

You just can't beat the 'feel' and control they give you with press priming.

rc

 

[243winxb]

I'm assuming sensitive primer

(CCI 400) You would be correct. Problem solved.

 

[Walkalong]

I have never had a slamfire in an AR using various primers. Granted, I do not shoot the number of rounds some of ya'll do. I did have a RRA two stage trigger start doubling. I replaced it and the problem went away. Seating primers below flush is paramount.

This photo shows that the hammer was not dropped and in contact with the firing pin.

How does it show that?

 

[243winxb]

How does it show that?

2 on left, primer flowed into the firing pin channel. The firing pin nose was not contacting the primer. The hammer was still cocked.

 

[edfardos]

oh, and for full disclosure, i've been proving the new Magpul 10-round magazines, which were definitely in play during the first slam fire, and possibly involved in the second. Yah, I'm from California, and have been patiently waiting for Magpul to produce a real 10-round mag.

--edfardos

 

[Walkalong]

2 on left, primer flowed into the firing pin channel. The firing pin nose was not contacting the primer. The hammer was still cocked.

Makes sense, even though a poor firing pin fit in the FP channel can cause the primer to look like the ones on the left. But that isn't the problem judging by the other cases fired in the same rifle. Interesting. Thanks.

 

[IMtheNRA]

That's the reason they have CCI-41 primers. Magnum and bench-rest primers are also thicker than standard CCI-SR primers.

A couple of years ago, I had a slam fire when using up a box of left-over CCI-SR primers and slam fired primer looked just like yours. It's not a big deal when plinking on the range, but could get me DQ'd from a match. Been using bench-rest primers ever since.

Had you doubled by accidentally pulling the trigger, regardless of the aftermarket trigger brand, the primer would look just like the rest of your normally fired primers.

Regarding high-primers, I think that's less likely to cause a slam fire than a misfire, since much of the firing pin energy would be used to seat the primer and crush the priming compound between the cup and anvil. Then, it would have to retain enough energy to activate the primer. Seems like a long shot to me.