Also, always feed the M1a/M14 from the magazine. A number of OOB's occurred when shooters placed the round in the chamber and dropped the bolt.
It must be understood that anything incidentally contacting the primer can cause cartridge ignition. This is due to the unpredictability of primers. It is fortunate that primers are as predictable as they are, but anyone who has ever shot enough ammunition can attest, you will run across the occasional dud primer. A dud primer is an overly insensitive primer and is very obvious by the fact it did not ignite. The overly sensitive primer is quite a bit more subtle in operation and as long as all primers go bang, just how do you sort out sensitive primers from normal primers? You can’t do that by a visual inspection. It is also not obvious but primers vary in sensitivity by brand, by lot, and within the lot. In the book:
The Grand Old Lady of No Man's Land: The Vickers Machinegun there is an interesting section about Vickers aircraft machine guns.. To raise the cyclic rate the British lightened the firing pin, but that caused misfires as the firing pin now had less kinetic energy. To fix that, the Brits used less sensitive primers, but now they had rounds ignite out of battery as the primers were being impacted within the mechanism (by something) before the breech was locked. It required a hardware change in the feed mechanism to fix that. It appears that machine gunners are very familiar with out of battery explosions:
http://www.ar15.com/archive/topic.html?b=6&f=2&t=145836
Any weapon with a floating firing pin, ie a firing pin that is not held back for the bolt face by use of a spring or some other mechanical device should use hard primers. CCI has a small primer called the 5.56. It is hard.
Chamber a round in your AR/M16 next time you are at the range and then unload that round. You will see a mark on the primer where the firing pin bounced.
Most all rifle caliber open bolt machineguns use a locking mechanism of one type or another so that in normal operation they cannot slam fire before the bolt is in the securely locked position. However firing pins break, firing pins jam in the out or fired position and most commonly cases separate leaving the case neck of the previous round in the chamber and in some weapons designs this can lead to a round firing out of battery. This type of a detonation can be anything from a minor irritation to destruction of expensive parts and injury to the shooter. I have experienced out of battery detonations in M60s, MG34s, MG42 and 1919A4s. Injuries were mainly cuts from brass case fragments and minor 1st degree powder burns. These are all pretty strong guns and damage to the guns varied from none to having to replace a couple of top covers.
Never use soft primers in any machinegun and especially belt feds.
ARs and M16 explode spectacularly. It is quite a sight. Serious injuries are quite rare with the AR/M16 although I can't explain why as in a lot of cases the upper seems to pretty well vaporize.
The Garand mechanism is an early semi automatic weapon design. It is my opinion that later mechanisms incorporated more thought into preventing incidental firing pin contact with the primer prior to lug engagement. As an example, AR mechanisms fully retract the firing pin until cam down. This is a very positive means of preventing firing pin initiated out of battery slamfires. If the firing pin is fully behind the bolt face till lug engagement, the primer is 100% protected from incidental contact with the firing pin. Credible out of battery slamfire incidents in AR’s are very rare. Armalite provides extensive large print warnings not to remove the firing pin spring in their AR10 actions.
Technical Note 10: Prevention of Slamfires explicitly states that the firing pin spring reduces the inertial impact energy to a very safe level and
almost always cures slamfires.
http://www.65grendel.com/forum/printthread.php?t=341&pp=10&page=2 ] I don’t believe their statement that Government and commercial large rifle primers are not “Hardened” in the same way as the Government #41 primer. (Based on talks with CCI, commercial large primers are more sensitive than the #34 primer) Armalite is correct that free floating firing pins will lightly impact a cartridge primer and, very rarely, cause the primer to ignite. Their technical bulletin advises to always feed from the magazine, because the friction of stripping a round from the magazine slows the bolt, and to never, ever, remove the firing pin spring. In
Technical Note 59: AR-10(T) Addendum to USMC M16A2 Technical Manual there is a picture of the Armalite spring on a firing pin and the large print notice:
”WARNING: THE FIRING PIN SPRING IS AN IMPORTANT SAFETY DEVICE AND MUST NOT LOOSE, REMOVED, OR LOST” From
Armalite Operator’s Manual http://usarmorment.com/pdf/AR10M15OperatorsManual.pdf
The AR-10 firing pin spring reduces the firing pin mark on the primer of a chambered cartridge that is chararacteristic of M-16 type rifles, and the “slamfire” possible with overly sensitive primers”. “THE FIRING PIN SPRING IS AN IMPORTANT SAFETY DEVICE AND MUST NOT BE LOOSE, REVERSED, REVOVED, OR LOST.”
Simonov’s SKS and the Fabrique Nationale FAL were extremely hard to find in the US prior to the Reagan administration, but since then they were imported in mass quantities. There is a domestic manufacturer of the FAL so supplies won't be as tight as they were prior to the 1980's. Both mechanisms are examples of designs that use a titling breech face to reduce the possibility of an out of battery slamfire. In both of these designs the bolt face is out of perpendicular from the case head until the bolt is in battery. This keeps the firing pin out of parallel with primer until the bolt faces rises as the bolt goes into battery. Both of these rifles have free floating firing pins, the FAL has a firing pin spring to reduce the chance of a slamfire and so do some models of Russian SKS’s. I never saw a Chinese SKS that had a firing pin spring and there are plenty of in-battery slamfires reports, most particularly the SKS, due to firing pin inertia with either of these models. Murray’s sells a SKS firing pin spring to reduce firing pin impact on bolt closure.
http://www.murraysguns.com/sksown.htm. There are a few firearm designs which don’t manually retract the firing pin and the SKS and FAL are among them. For these designs it is safety critical to keep the firing pin channel clean to prevent the firing pin from being wedged in a forward position. Here, Murray performs an insanely risky test with an SKS. He has wedged the firing pin forward and the gun fires until the rifle is empty.
https://www.youtube.com/watch?v=xj3QtnUWCwQ The primary reason the rifle does not have an out of battery slamfire is due to the titling breech face design of the mechanism, but given a long enough firing pin, I don’t see any reason why an out of battery slamfire could not happen. Here is a video of an SKS which the owner claims is well maintained and it slamfires in battery.
https://www.youtube.com/watch?v=mPn97vz0Vyw
D.J Saive, the designer of the FAL, in the earliest version, the FN49, inside the patent there is an explicit claim for a firing pin blocking feature.
Tilt Locking Breechblock for Automatic Firearms D. J. Saive July 18, 1950, 2,515,315
The spring 32 also forms a safety device (Figure 14) for preventing firing when the breech block is unlocked. As shown in 'Figure' 14, the firing
pin 31 has a groove 31a in it and the spring 32 has a lug 32a on it which acts as a safety and, by engaging in the groove 31a, holds the firing pin against forward movement when the breech block is unlocked . When the breech block is not locked (Figure 8) the rear end of the slide projects beyond the rear end of the firing pin thus preventing the striker from striking said firing pin.
You can see a picture of the firing pin safety stop here:
http://milpas.cc/rifles/ZFiles/Semi-automatic Rifles/FN 59/bolt disassembly.htm
Unfortunately, as anyone who has searched, the firing pin safety stop has not been positive in function as there are lots of accounts of out of battery slamfires in the FN 49. One cause is a weak firing pin that breaks and sticks out through the bolt face, but there are a number of accounts where the firing pin was found to be intact and not stuck within the barrel channel. These posts have some interesting pictures:
Kaboomed 8mm Egyptian FN-49 .... Now A Floorlamp!!!!
http://www.sksboards.com/smf/index.php?topic=132165.0
Argentine 49 7.62x51 KABOOM!
http://www.surplusrifleforum.com/viewtopic.php?f=43&t=106950
Something must have changed with the FAL, while in battery slamfire accounts are easily found, out of battery slamfire accounts are rare.
As can be seen with this FAL bolt, when it is unlocked, the firing pin center of axis is off set from the primer center. Any incidental firing pin contact is less likely to ignite the primer. Data shows that the further the firing impact is from the center of the primer, the greater the misfire rate. Primers need to be hit in the middle for positive ignition.
Another feature is that the breech face is out of perpendicular with the cartridge base. Only at lockup does the bolt face and firing pin square up. There could be out of tolerance conditions that could create incidental firing pin contact with the center of the primer, but in this mechanism, such events are rare.
FAL Bolt out of battery
FAL Bolt in battery
I am impressed with the HK roller bolt actions. The primary concern of the German designers was to create a rifle that could be quickly and inexpensively mass produced, a goal they achieved, and they also produced an easy to maintain and safe rifle. I have never heard of any in battery or out of battery slamfire events in the roller bolts and I am of the opinion that a firing pin induced out of battery slamfire is totally impossible in this design without part breakage. The firing pin absolutely cannot move forward of the breech face until the rollers are in battery:
These are pictures of the bolt mechanism of my PTR 91, you can see the firing pin spring, “connecting rod” and bolt.
The firing pin mainspring is very strong, much stronger than I have found on any of the actions I own. To call this firing pin “free floating” is almost an oxymoron as spring tension must keep the firing pin in place regardless of firing pin inertia.
Of the military actions on the market, the Garand mechanism has the most reports of slamfires. One reason has to be the hundreds of thousands of Garands that have been imported and are now in civilian hands. Another reason is that the Garand action has a long free floating firing pin. This mechanism does not positively hold the firing pin throughout the feed cycle and the firing pin is always in line with the centerline of the cartridge case. Due to these characteristics the Garand mechanism will not only slamfire in battery, but out of battery. I am of the opinion that the US Army controlled slamfire rate in the Garand (and M14) by specifying a relatively insensitive primer. I am of the opinion this was how most militaries controlled slamfires in their issue rifles: primer insensitivity. On page 58 of the April 2011 Guns Magazine Mike Venturino reports having a in battery slamfire in a K43 and SVT40 rifle with standard primers. These slamfires ended when he used #34 primers.
