Slam Fires and the AR Platform

A "slam fire" is when the round being chambered fires when the bolt closes.  There have been some reports of these in ARs, but with surprisingly few details provided.

Slam fires are usually the result of high primers and possibly long firing pin protrusion. High primers just aren't in reloads, they happen in factory ammo too. I have several factory .223/5.56, .308, .45ACP round with what I'd call high primers. However, I bet many of "slam fires" are actually caused by having fingers on the trigger when dropping the bolt.

I decided to look into the matter.

First we need to get some terminology straight.  Some folks think that "hardness" refers to the power level or flame of the primer which is correctly called brisance."   In this discussion, "hardness" refers to the physical toughness of the primer cup.  

The Test

Firearm used was a Bushmaster Patrol Carbine with a verified (by chamber cast) military 5.56 chamber (print 8448549 / 8448550, without the double shoulder angle). Headspace was nominal, and the action spring met military criteria. Firing pin protrusion was .031". Carbine was equipped with an H2 buffer.

New, uncrimped, LC 11 cases were used and they were checked in a case gage for proper base to shoulder length.  Primers were seated using a Bonanza CoAx press's positive seating depth primer seater (seats .005" below case head).  Groups of five primed cases each had different primers seated in them.

Bolt was locked back without a magazine in the carbine and the primed case was dropped into the chamber.  With the muzzle pointed about 20 deg below horizontal and the carbine held only by the pistol grip, the bolt was released by pulling fully back on the charging handle and releasing (to get maximum bolt velocity without magazine drag in feeding).

The average of the diameter of the firing pin indent was measured for each primer using a 10X measuring microscope with a reticle. (Within each group the indents were effectively identical.)

The case with the largest indent in each group was then subjected to 4 additional bolt drops as described above and the indent then measured.

The results are tabulated below.

Brand Avg Single Indent Multiple indents
(5 loadings)
CCI 400 .026 .039
Federal 205 .025 .038
Remington 7 1/2 .026 .039
Winchester SR .029 .042
CCI 41 .026 .038
Federal 100 SP (Included for comparison.  Not a rifle primer) .030 .044

While I don't have the time to do a similar test with large primers I am very sure the results would be similar.  However, the M1 Garand, M1 carbine, and M14 are all designed to cam the firing pin back and prevent it from moving forward unless the bolt is fully closed.  With these arms slam fires require a very high primer to occur.

Some Additional Thoughts to Think About

If you use commercial cases, primer seating is not a big deal. If you use GI cases you need to swage the crimp out of the primer pocket AND ensure that there is no trace of the crimp jammed down into the primer pocket.  In setting up for this test I played with some crimped once-fired GI cases that were swaged with a Dillon primer pocket swager. While the crimps were nicely swaged out, in many instances there was a sliver of the crimp stuck in the primer pocket that would interfere with primer seating resulting in high primers.

To check out a worst case scenario, I swaged some primer pockets of a bunch of once fired LC08 cases, until I got 10 cases with the sliver in the bottom. I hand primed these with the Win SR primer with a Lee hand tool until the primers FELT seated and wound up with 5 cases each, with the primers approximately .001" and .002" above the base of the case. I then dropped the bolt on each of them as described above. None of the .001" high primers fired.  With the .002" primers, one fired the first time, and 1 required 3 drops to go off.  The indents on the cases that fired were about .047 - .048.  The remaining three survived 5 drops without going off but had large dimples in them (.045-.046)

My Conclusions. (YMMV)

If you are loading for the AR and are concerned about slam fires ...

1) Verify your firing pin protrusion (the spec is .028 - .036, with the ideal being .030 - 032) and ensure that the firing pin moves freely in the bolt without sticking.

2) Preferably use CCI 41 or Federal 205 primers, but anything but the Winchester small rifle will probably be ok if you are careful.  No matter what primers you use inspect ALL your primed cases to ensured the primers are seated below the base of the case.

3) If using cases with swaged primer pockets ensure there are no stray pieces of crimp in the pockets and that the pockets are clean.  I have found that using the "steel" pin media (used wet) cleans the pockets out very well and smoothes the edges a little.

4) Ensure you are using the correct shell holder for your caliber and priming tool.  An incorrect shell hold can prevent the primer from fully seating.

Thanks to David P. for reminding me of this.

5) Watch your reassembly.  A correspondent noted a slam fire because of incorrect assembly.  It seems like the legs of the firing pin retainer cotter pin spread when it was inserted so both legs didn't go through the hole on the opposite side of the carrier. One leg bent down and kept the firing pin forward which caused a slam fire when the bolt was dropped on a live round.  Something to pay attention to.  Thanks to Arnold M. for this tip.

6) Especially remember Rules 2 and 3. (For those who aren't Gunsiters here are The 4 Rules.

  1. All firearms are loaded. - There are no exceptions. Don't pretend that this is true. Know that it is and handle all firearms accordingly. Do not believe it when someone says: "It isn't loaded."
  2. Never let the muzzle of a firearm point at anything you are not willing to destroy. - If you are not willing to see a bullet hole in it do not allow a firearm's muzzle to point at it. This includes things like your foot, the TV, the refrigerator, the dog, or anything else that would cause general upset if a hole appeared in it.
  3. Keep your finger off the trigger unless your sights are on the target. - Danger abounds if you keep your finger on the trigger when you are not about to shoot. Speed is not gained by prematurely placing your finger on the trigger as bringing a firearm to bear on a target takes more time than it takes to move your finger to the trigger. Negligent discharges would be eliminated if this rule were followed 100% of the time.
  4. Be sure of your target and what is behind it. - Never shoot at sounds or a target you cannot positively identify. Know what is in line with the target and what is behind it (bullets are designed to go through things). Be aware of your surroundings whether on a range, in the woods, or in a potentially lethal conflict.

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As far as I know all the information presented above is correct and I have attempted to insure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for you doing something stupid with it. (Don't you hate these disclaimers? So do I, but there are people out there who refuse to be responsible for their own actions and who will sue anybody to make a buck.)

Updated 2015-03-23