When I bought my SuperMatch M1a’s, Springfield Armory provided a copy of Wayne Fattz’s article
“The Mysterious Slamfire” which was printed in the American Rifleman in Oct 1983.
http://www.scribd.com/doc/2649554/The-Mysterious-Slamfire-
Mr. Faatz had an out of battery slamfire, from the clip, with Federal primers. He wrote an article trying to understand why it happened to him.
First on his list of slamfire causes is a sensitive primer.
1. Sensitive Primer (included under this category is a high primer)
2. Minimum headspace chamber and inadequate case sizing (leading to the firing pin hitting the primer with excessive force)
3. Hammer following the bolt
4. Fouled bolt face
Mr Faatz ran a simple experiment dropping the bolt on primed cartridges. He was unable to have a high primer ignite until he inserted a spacer under his primer. This confirms CCI’s statement that high primers are the most common cause of misfires:
Mysteries And Misconceptions Of The All-Important Primer
http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/
Page 5 of 9
The real story is that Boxer primers leave the factory with the anvil higher than it would be when seated in a cartridge case. Seating so anvil legs touch the bottom of the pocket lets the anvil tip penetrate into the pellet of mix. The nearly universal recommendation of having the primer cup bottom 0.003 to 0.005 inch below flush with the case head exists to set the proper amount of priming mix between the cup and the anvil tip.
This critical distance is known as the bridge thickness. Establishing the optimum thickness through proper seating means the primer meets sensitivity specifications but does not create chemical instability. However, failing to set the bridge thickness through proper seating depth is the number one cause of primer failures to fire. The bridge thickness is too great with a high primer, even one whose anvil legs touch the bottom of the pocket
Once Mr. Faatz put a spacer under his primer he met the conditions of a high primer, a firmly seated anvil and the anvil pushed into the primer cake.
SKS’s slamfire so often there are lots of slamfire reports with SKS’s, Murray’s has a firing pin modification to reduce the chance of slamfires. Also, the SKS boards has this excellent “A primer on primers”
http://www.sksboards.com/smf/index.php?topic=56422.0
And for those who can read,
Springfield Armory M1A Manual, page 4
www.springfield-armory.com/download.php?asset=M1AManual.pdf
Ammunition
The M1A is designed and built to specifications to shoot standard factory military 7.62 NATO ammunition. The specifications for standard military ammunition include harder primers to withstand the slight indentation from the firing pin when the bolt chambers a cartridge. This slight indentation is normal. The use of civilian ammunition with more sensitive primers or hand loads with commercial primers and/or improperly seated primers increase the risk of primer detonation when the bolt slams forward. This unexpected "slam fire" can occur even if the trigger is not being pulled and if the safety is on. Use of military specification ammunition will help avoid this.
Every shooter should use extreme caution when loading this or any other firearm. See page 17 for instructions on proper loading to help avoid a "slam fire". Also see enclosed article on “Slam Fire” written by Wayne Faatz
There are important safety considerations when reloading for Garands, M1as, and M1 Carbines. All of these rifles are functionally similar in that they all have a free floating firing pin. The risk in all mechanisms with free floating firing pins is that incidental contact between the firing pin and primer can cause cartridge ignition, or a “slamfire”. You can search the web and every mechanism with a free floating firing pin has a posted slamfire incident. This includes AK47s, SKSs, French rifles, etc, etc, etc. The worst case slamfire is a slamfire that occurs out of battery, this will result in a catastrophic failure of the mechanism. The Garand mechanism has the most out of battery slamfires of any rifle on the market, and this is due to the lack of a positive mechanical firing pin lock or blocking device. All of the Garand based mechanisms share a firing pin retraction cam, this cam pulls the firing pin back during extraction. It also has limited utility as a safety device, because it blocks to a limited extent the firing pin going forward, but that only happens in the last 0.007” of movement. Long or tight rounds totally defeat the firing pin retraction’s cam ability to hold back the firing before cam down. Given an overly sensitive primer, one that will go off when struck by a rebounding firing pin, if you sized your round larger than the chamber, you will have created the highest risk conditions for an out of battery slamfire.
Only at final cam down is the firing pin retracted. Up to then the firing pin is totally free floating and tapping the heck out of the primer.
This is a M1 Carbine firing pin retraction cam.
A M1 Garand firing pin retraction cam is functionally identical, just the carbine is easier to visually understand.
Firing pins in this mechanism are free floating and the mechanism does not have a positive mechanical interlock to prevent incidental contact between the free floating firing pin and the primer. As can be seen here, that bolt has around three inches of travel before the firing pin tang touches the retraction cam.
This is a M1 Garand receiver and the firing pin is fully forward and just touching the firing pin retraction cam. As you can see there is only thousand's of an inch of forward movement left in bolt cam down and yet the firing pin is out about 0.064" of the bolt face.
It turns out that cam down is the point at which out of battery slamfires occur. It makes sense that it is here because the bolt will be at its highest forward velocity and yet, it has to de accelerate to go into battery. The firing pin , of course, has the same velocity of the bolt, but on cam down when the bolt stops, it continues to move forward and it will rebound off the primer. If the bolt has to stop to crunch fit a long or fat case, that firing pin is rebounding off the back of the primer and this is happening when the lugs are not engaged.
That is why it is best practice to small base size cases used in these rifles and to set up the dies with a case gage and size to gage minimum. (Assuming you don’t know the headspace of your chamber, if you do, always be 0.002 to 0.003” less) You want the bolt to close without resistance. This will reduce the risk of an out of battery slamfire assuming an overly sensitive primer. You can still have an inbattery slamfire given an overly sensitive primer, an undesirable event, but an inbattery slamfire is less likely to blow off the back of your receiver, blow the bolt face off, rip off the elevation knob, and split the stock in two.
If you attempt to small base size with a spray on lube you will stick the case in the die. I recommend RCBS water soluble or Imperial Sizing wax. These are excellent lubes.
For these rifles it is good practice to ensure ream primer pockets to depth, seat the primers by hand, and verify that all of the primers are below the primer pocket. There is a chance that a cocked primer, with the anvil firmly seated on something, could ignite when hit by the bolt fact. Clean those pockets! High primers are one of the most common cause of misfires because the primer won't fire unless the anvil is seated and is pushed up into the primer cake.
http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/ However, given a shallow pocket, given debris in the pocket, it is possible that high a primer could be ignited by the bolt face.
After seating primers examine the back of the case and see if there are high or cocked primers.
It is also safety critical to use the least sensitive primer around because these rifles will slamfire in battery or worse, out of battery, given a sensitive enough primer.
Federal primers are the most sensitive primer on the market and the most "slamfiring" primer in Garands. I have lots of web accounts of slamfires with Federal primers. Don’t use them. I recommend CCI #34s and Tula7.62 primers as they considered "Mil Spec" primers. Which means they are less sensitive than commercial primers, federal are the most sensitive commercial primer on the market
Slamfires are a low probability event, but they do happen. I have a paper copy of this document :
Comparison Test of Rifles 7.62 mm, M14, Manufactured by Springfield Armory and Harrington and Richardson Arms Company, July 1963 . This has the only credible data that I have ever seen which can be used in estimating out of battery slamfire probabilities. In this test six new M14’s were undergoing various tests. Each was to be fired 6000 rounds each. One Springfield Arsenal M14 slamfired out of battery at round 5271. Based on this, I estimate that the probability of a slamfire with military spec primers is 1:35,000 per round fired. The slamfire risk with more sensitive commercial primers would be higher.
This was an interesting slamfire and it was with a military M14.
Help me figure out if this is a slam fire or bad loading. KABOOM!
http://www.thehighroad.org/showthread.php?t=767168&highlight=m14
A summary of the event is that the shooter used new, unfired brass and primed the cases with CCI Benchrest primers. All "match" or "benchrest" primers are too sensitive for this mechanism. His rifle slamfired during cam down. You can see the damage to the rifle and a video link. He was lucky, there was partial lug engagement, if the primer had gone off out of battery, and the bolt blown the back of the receiver off, he might have lost a $20,000 dollar rifle.
When firing single shot, use a SLED. Do not put a round in the chamber and drop the bolt. Lots of inbattery slamfires, and a few out of battery, have happened because of this. You want to slow the bolt down. When rounds feed from the clip the friction between cartridges slows the bolt a bit. You want to use a SLED as that slows the bolt a bit.
If loading a Garand without a SLED, lower the bolt about 1” over the follower and let go. Always get your hand out of the way so that if the mechanism slamfires the operating rod does not cut through the palm of your hand.
My advice for case preparation for Garands/M1 as to
1. Set up dies with a case gage.
2. Full length resize in a small base die
3. Trim cases
4. Clean primer pockets, ream to depth
5. Prime all cases by hand, verify that all primers are below the case head, and use the least sensitive primers you can find.
If you can’t find CCI #34 or Tula7.62 primers then you have to use primers of commercial sensitivity and the risk of a slamfire increases, but I don’t know how much. I do not recommend Federals in this mechanism, I don’t recommend the new brass colored WLR, CCI 200 seems to have a harder cup than other commercial primers, I have never tried Remington primers and have no opinion. I have conducted my own primer tests to see what happened over the chronograph, personally I am a fan of the Russian primers, they shoot very well.
A source for the old NRA recommended loads, for bullet weights other than 168 or 174 can be found here.
http://masterpostemple.bravepages.com/M1load.htm I load my 150’s with 47.5 grains IMR 4895 instead of a max recommendation of 49 grains. I have chronographed and tested all my loads, a 150 SMK with 47.5 grains IMR 4895 is moving just at 2725 fps, and that is as fast as I want a 150 grain bullet to go in this mechanism, regardless of what someone else may say.
Check cartridges for case head separation. Gas guns are hard on brass: the bolt unlocks while there is still significant chamber pressure. Because of this the case gets stretched on extraction. Carefully inspect cases for stretch ring marks at five reloads. They occur about .4” of an inch ahead of the base. You can verify if the cases are internally necking by inserting a bent paperclip in the case, and feeling for an edge.
The Italians added a firing pin spring to their BM59’s, obviously to reduce their slamfire rate. Because I am concerned about slamfires, I had two out of battery slamfires with Federal Match primers, I sent the bolt of my match Garand to Roland Beaver to install a similar spring.
Roland charged me $25.00 total, I shipped it on a Monday, received the bolt back next Monday.
This has not eliminated firing pin indentations in my Garand. As a worse case test, I placed these rounds in the chamber, pointed the muzzle at the ground, and tripped the bolt release. Even with a firing pin spring, these primers were dented.
I “believe”, “hope”, and “pray”, that the spring is reducing the kinetic impact energy of the firing pin, but without instrumentation, I don’t know. It should, but I don’t trust it enough to use Federals or any of the other “more sensitive” primers in my Garand
References:
a. The Mysterious Slam-Fire, American Rifleman, Oct 1983
b. Reloading for the M1 Rifle, American Rifleman, Mar 1986
