Is it possible that the primer was a bench rest style primer and I got a slam fire?
My vote is out of battery slamfire due to using primers that are too sensitive for the mechanism. Bench rest primers have thinner cups than mil spec, possibly a more sensitive primer mixture, and likely have a different anvil. US military spec primers have prescribed sensitivity requirements that were tailored to this mechanism. The Army developed a specific primer mix, cups are thick and harder and the anvil angle is different, to absorb more energy and thus make the primer less sensitive than the average commercial primer. Older shooters automatically respond with a root cause of high primers, because that is what they were taught. However, high primers are the most common cause of mis fires, not slamfires. Basically, if the anvil is dangling in the air and the primer cake not set against the anvil, what will result is a misfire.
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
Conditions can be created that would induce a high primer slamfire, and that requires placing a spacer under the primer or having a shallow primer pocket. These things can happen, and both fulfill the requirements of a firmly seated primer and a proper setting of bridge thickness. However in this incident, having factory new brass, it is very unlikely to have shallow primer pockets. Regardless, I recommend reaming primer pockets to depth, hand seating all primers, and verifying that all seated primers are below the case head. While high primers are the most blamed cause of out of battery slamfires, in my opinion, they are not the most frequent cause of slamfires, in fact they hardly ever cause slamfires, the primary cause of slamfires, both in battery and out of battery, ignoring faulty trigger groups, is sensitive primers.
The proper primers in this mechanism are the military spec primers, basically CCI #34’s or Tula7.62 primers. These primers are less sensitive than commercial and reduce the changes of a slamfire, but they don’t eliminate the possibility. Primers vary by sensitivity within the lot, lots vary by sensitivity, and so do primer brands. Federal is proud to make the most sensitive primer on the market and Federal match primers are the most slamfiring primer on the market. Federal does offer a “mil spec” small rifle primer, for the AR shooters, and we ought to encourage all primer manufacturers to offer their mil spec line to the public by not using their commercial primers in Garands mechanisms.
The basic problem in the Garand mechanism is that it relies
solely on primer insensitivity to prevent slamfires. Firing pins in this mechanism are totally free floating and fully able to rebound off the primer through most of the forward bolt cycle. There are many that tout the firing pin retraction cam (or receiver bridge) as a positive barrier, something it is not, and in my opinion, never was. The firing pin retraction cam is ineffective as barrier, only contacting the firing pin tang during the last 0.007" forward movement of the bolt, and, in my examination of three stripped Garand receivers, for all of the receivers, the firing pin tip is forward of the bolt face through out cam down. Given a long or a fat cartridge, the bridge is definitely not engaged, and given bolt bounce, chatter in cartridge chambering, the firing pin can hit the primer when the lugs are not engaged. This could have happened to you since you had not full length resized that brass. You were just assuming that the brass was not bulged or over length, and it could have been.
Or not, either way, you were using the wrong primers.
These rifles have been slamfiring out of battery since the day they were made. If you use them, you have to recognize that sometimes, you can do everything right, but the risk exists, you have can have a combination of aggravating factors, starting with the primary requirement of an overly sensitive primer, and the mechanism will slamfire.
I have a paper copy of this report. The Army was testing production models of H&R’s and using SA M14’s as control specimens. Six rifles were under test, three by HRA and three by SA. All rifles were gaged for dimensional part compliance, and part interchangeability. Such things as the thickness of the chrome coatings were measured, etc. After gauging the rifles were reassembled and underwent endurance testing which was each rifle was to fire 6000 rounds. The total amount of 7.62 Nato to be fired would have been 36,000 rounds.
USATECOM Project No 8F-3002-04, Comparison Test of rifles, 7.62 MM, M14 Manufactured by Springfield Armory and Harrington and Richardson Arms Company. Author G. E. Hendricks, July 1963.
At round 5271 a Springfield Armory M14 went off out of battery. The report states:
“
One rifle fired when the bolt was in the unlocked position causing breakage for the firing pin, extractor, bolt roller, ejector, and stock. The magazine split, causing the magazine floor-plate spring and 12 rounds of ammunition to be ejected against the bench rest from which the rifle was being fired. The case ruptured and several pieces of brass were found in the area. A broken part of piece of brass perforated a cardboard box with was position between the gunner and the proof director. The cardboard box was used as a brass catcher. Not all the broken pieces were found. Although no one was physically injured this is a seriously unsafe condition.”
So, six rifles were tested, five completed the test firing 6000 rounds each, one slamfired out of battery at round 5271, for a total round count of 35, 271. Therefore a crude estimate for the probability of an out of battery slamfire, with mil spec primers, is 1:35,000. Since commercial primers are even more sensitive, on the average, than mil spec primers, the probability of a slamfire may be 1:9000, or even 1:900. And this is assuming that the primer manufacturer follows their quality control procedures and does not ship non conforming materials, that is, an overly sensitive lot of primers. Since no one has any visibility to what these guys are doing, you can assume that they ship defective material because no one sues and thus there is no cost liability.
For added anti slamfire insurance, I sent a match 30-06 Garand bolt to Roland Beaver and he added his firing pin modification. The Italians did this to their BM 59’s, obviously they decided one slamfire is one too many slamfires. This spring will reduce the firing pin impact energy on the primer. This firing pin spring is strong, but even with a spring, I get firing pin dents on chambered rounds.
Roland charged me $25.00 total, I shipped it on a Monday, received the bolt back next Monday.
As for the belt on your cartridge case, I am of the opinion that the case head was not fully supported because the cartridge was not fully seated. Obviously the sidewalls ruptured as pressure built. Cartridge head protrusion is an extremely important dimension and if too much case head sticks out, the sidewalls will rupture.
These pictures came from Chinn's book the Machine Gun, Vol IV. I recommend that everyone read the blowback section to understand case head protrusion.
Longdayjake: I would like permission to use your pictures in discussions about out of battery slamfires.
