How true is the whole Garand slam-fire theory?

[DWARREN123]

Usually with a semi-auto rifle a slam fire is caused by a dirty or damaged firing pin/bolt combination.

[Patrick Carr]

There is a big difference in a slam fire when the bolt is in battery, vs when the bolt is out of battery. I had the unfortunate experience of the latter.
I blew up a perfectly great IHC DCM Garrand in 1986 by not checking headspace on my reloads. I was using CCI-250's, but away it went anyway.
My problem was not the primer.
The LCM-72 cases had about 5 cycles on them, and my RCBS regular, NOT small-base, die was sizing a little bit short, I found out.
After recovering, and obtaining a new barreled action from the DCM, I got a Mo's headspace mic, which looks just like the new RCBS gage. I compared it with my local smithy's gage, amd the Mo's was off by .001". OK, so noted. I checked some of the rounds from the fateful batch, and guess what? They were about .005 too long. SO, I faced off my RCBS die until it was sizing right at + .001 on my gage, which in the real world = GO headspace.

Don't take your reloads off the bench without running every round through one of these. Also run your finger over the primer, checking for below flush.
If you don't have time for these quality checks, you should not be exposing other shooters on the line to your carelessness. These steps take a lot less time than rebuilding a rifle. I still have walnut coming out my fingernails.
Harder primers are a good idea too, but that is another story.

Patrick Carr

[Mike Irwin]

One other apparent cause of slam fires in rifles like the SKS has been the KIND of lubricant used on the firing pin.

When I was working for NRA SKS rifles started coming into the country in droves, and we started getting reports of slam fires.

After talking to a bunch of owners who had had this happen, we found a couple of things were very common across the slamfire incidents...

1. Total disassembly of the bolt to remove cosmoline.

2. Lubricating the firing pin with one of the super-slick teflon or moly disulphide light-bodied lubricants (not grease).

3. American-made commercial ammunition, which typically has far softer primer cups that military ammunition.

Those factors combined to make a slam fire a LOT more possible.

The salient point is DO NOT USE A "SUPER LUBRICANT" ON THE FIRING PIN.

It apparently reduces friction so much that firing pin velocity is sufficient to set off the softer American commercial primers.

[RonC1]

I don't understand the comments about HIGH primers. In my experience, a round with a high primer is much more likely to misfire than to fire unexpectedly. The firing pin likely just seats the primer on the first strike, rather than igniting it.

Comments? What am I missing?

[Sport45]

With a high primer it's not just the firing pin, but the bolt itself that can slam into the primer when chambering. When single-loading a garand you drop the shell in front of the bolt, drop the bolt by hand about half way and then release it to let the spring finish the process. If you release the bolt from all the way back you slam into the cartridge pretty fast.

[Slamfire]

I

was using CCI-250's, but away it went anyway.
My problem was not the primer.
The LCM-72 cases had about 5 cycles on them, and my RCBS regular, NOT small-base, die was sizing a little bit short, I found out

.

The primary cause of slamfires is primer sensitivity. If the firing pin has sufficient energy to ignite the primer, the primer is going to ignite.

Primers vary in sensitivity, how much, I don't know. Primer manufacturers don't release the data.

Your over long cases increased the chance of an out of battery slamfire. Did not change your primer sensitivity.

When brass is a little too long, your mechanism has enough energy to crunch the brass to fit. Unfortunately that pushes the base of the cartridge firmly into the bolt face. Before the lugs start to cam, that free floating firing pin is just tapping the heck out of that primer.

If you have a sensitive primer you have just increased the chances it is going to get a hard enough tap to set it off.

By the way, slamfires happen in AR's. Usually when loading the rifle single shot. Generally in AR's there is some lug engagement when this happens and the rifle is not damaged.

Slamfires have happened at the John Garand Match at Camp Perry. I heard it, everyone heard it and a bud was nearby. It was in 2002 or so, the first year they issued Federal manufactured ammunition. If you don't know, Federal primers are the most sensitive primer out there. It was during the load command during the standing stage. The unfortunate placed a round in the chamber and dropped the bolt. The round went off. My bud would have told me if the gun or shooter was damaged, so it must have been in battery.

Some folks, you just cannot convince them that slamfires are real. They will come up with every cause to explain what happened except primer sensitivity. Some are valid. But slamfires have happened, do happen, and will happen. You just don't want to have it happen in a Garand type mechanism.

[totalloser]

I have experienced slam fires in an AR. In the form of double taps. Factory trigger, cheap (non wolf) ammo. I also doubt that a high primer will do it; Usually the chamber will not lock up if the primer sticks out too much, and I know for a fact that dead on hits from 9mm won't set off primers. At least not often enough to happen when you try. (everyone is young and dumb... really dumb once) So if a 115 grain bullet won't set it off, I doubt a bolt face could.
The thing to worry about it a slam fire lighting one off before the bolt is locked up. I'd rather have it happen in a Garand than a AR. If it's anything like a Mini 14, the op handle will hit the front of the reciever and stop the bolt. Unless something breaks, that is.

[Sport45]

I know a BB out of a Crosman 760 air rifle will set off a shotgun primer. Still have the scar on my side from the primer demonstrating that "equal and opposite reaction" thing.

[amamnn]

Slam fires do happen. Until last year I had never witnessed one or knew anyone who had.

We had two separate incidents at two clubs to which I belong. I was on the firing line when one happened in an M-1. Both happened to handloaders who each had less than a year's experience loading for military style semi auto rifles. Both had loaded high primers. One had assumed he was safe and needn't uniform primer pockets because he was using the CCI "military" primers. A point to consider: if the military uses those primers on all its ammo, then why isn't there a shortage of only those primers?

A very good essay concerning the differences in loading for these rifles as opposed to bolt and other rifles:

http://www.exteriorballistics.com/re...sgunreload.cfm

[Unclenick]

Originally Posted by Slamfire

When brass is a little too long, your mechanism has enough energy to crunch the brass to fit. Unfortunately that pushes the base of the cartridge firmly into the bolt face. Before the lugs start to cam, that free floating firing pin is just tapping the heck out of that primer.

I think there is another critical detail that bears separate mention. In order for that tapping you mentioned to occur, the receiver bridge cannot yet have blocked the 'foot' of the firing pin. On the long case, it won't have completely done so. You can minimize the chances of such an event by checking that firing pin extrusion is within specifications, that the firing pin is within specifications from the tip to the inside of the firing pin foot, and that the receiver bridge is straight and undamaged and within specification as to its distance from the front of the receiver 'ring' surface that the barrel shoulder touches down on. If all these conditions are met, you should have at least a small margin of error to avoid out-of-battery slam fires.

It should be noted that minimum headspace match chambers increase the risk, since the shoulder of a long case will be kept even further to the rear by it. It should also be noted that a rapidly extracted case can stretch, and that you should not presume what ejects from your self-loader will accurately reflect your chamber's headspace. To get that information from the Garand, unscrew the gas cylinder plug and operate the op-rod manually to chamber a round. Fire it with the bolt carefully and fully closed forward, then extract it slowly by hand. That gently handled case will make a good headspace comparison basis, and all your loaded rounds should have headspace at least 0.002" shorter than it. Check this with the gage of your choice.