View Full Version : Primer marks from loading?
Is it normal to have firing pin marks on the primer just from loading?
The left one chambered then I removed it w/o pulling trigger. The right one was fired.
56756
Yes, it's normal with the free float firing pin used in the original design. I believe military ammo uses harder primers for just this reason. Some other brands use a firing pin spring to prevent this from happening. I have no idea why Eugene Stoner didn't. Maybe it would have complicated timing for full auto firing?
Some people rotate the first round in a magazine to avoid having it happen multiple times to the same round.
Perfectly normal. If you load the same round on a regular basis, rotate it deeper into the magazine. However, if you load that round often, check it to ensure there is no bullet setback while you are at it.
Here is some info from a police rifle instructor course:
The AR15 has a free floating firing pin. As a round is chambered the forward movement of the bolt transfers to the firing pin, allowing the firing pin to ‘lightly’ impact the primer of the chambered round as the bolt locks into battery.
An inspection of unfired rounds which have been extracted and ejected from an AR15 will reveal a light ‘witness mark’ on the primer. There have been reports that repeated chambering of the same cartridge can result in failures to fire, and in some cases, slam fires as the bolt goes into battery:
1) This past fall, a large SWAT team here had a failure to fire from an M4 with (redacted) ammo during an entry--fortunately no officers were hurt and the suspect immediately threw down his weapon when the carbine went click instead of bang.
The round from the potential OIS incident had a primer strike of only .013"—the minimum firing pin indent for ignition is .017". In addition, the primers on the other rounds were discovered to have been damaged from repeated chambering. When the same cartridge is repeatedly chambered in the AR15, the floating firing pin lightly taps the primer; with repeated taps, the primer compound gets crushed, resulting in inadequate ignition characteristics--despite what appears to be a normal firing pin impression.
2) From 1965: A soldier in the 82nd, during the operations in the Dominican Republic, chambered a round that had been chambered and ejected several times while preparing for guard duty . His weapon discharged, even though it was on "SAFE" according to witnesses at the scene. The soldier was negligent about where his weapon was pointed and the round killed a local child.
The soldier was court-martialed and acquitted. Investigators were able to repeatedly duplicate the incident by chambering a round from the magazine several times. They discovered that this malfunction, firing when a round was chambered, could be duplicated at around 10-15 chamberings. They also found that at about 10-15 chamberings, many rounds did the opposite and the primers became desensitized enough that the rounds would not fire.
3) ……I went through my duty ammo and pulled a couple of rounds that were dimpled. I took one of those and loaded it a total of 13 times then fired it. I pressed the shot heard a click...Boom. The delay was quit short, less than a second, but noticeable. I have heard of this, slow burn, but never experienced it before.
Ok, never looked so this is new to me. Thank you much for the information.
Being an engineer this makes me question the design as this is an obvious failure mode. Interesting.
Ok, never looked so this is new to me. Thank you much for the information.
Being an engineer this makes me question the design as this is an obvious failure mode. Interesting.
By the same token, would it be wrong to think of it as ammunition being used outside of normal parameters - being chambered and ejected repeatedly?
Here is some info from a police rifle instructor course:
The AR15 has a free floating firing pin. As a round is chambered the forward movement of the bolt transfers to the firing pin, allowing the firing pin to ‘lightly’ impact the primer of the chambered round as the bolt locks into battery.
An inspection of unfired rounds which have been extracted and ejected from an AR15 will reveal a light ‘witness mark’ on the primer. There have been reports that repeated chambering of the same cartridge can result in failures to fire, and in some cases, slam fires as the bolt goes into battery:
1) This past fall, a large SWAT team here had a failure to fire from an M4 with (redacted) ammo during an entry--fortunately no officers were hurt and the suspect immediately threw down his weapon when the carbine went click instead of bang.
The round from the potential OIS incident had a primer strike of only .013"—the minimum firing pin indent for ignition is .017". In addition, the primers on the other rounds were discovered to have been damaged from repeated chambering. When the same cartridge is repeatedly chambered in the AR15, the floating firing pin lightly taps the primer; with repeated taps, the primer compound gets crushed, resulting in inadequate ignition characteristics--despite what appears to be a normal firing pin impression.
2) From 1965: A soldier in the 82nd, during the operations in the Dominican Republic, chambered a round that had been chambered and ejected several times while preparing for guard duty . His weapon discharged, even though it was on "SAFE" according to witnesses at the scene. The soldier was negligent about where his weapon was pointed and the round killed a local child.
The soldier was court-martialed and acquitted. Investigators were able to repeatedly duplicate the incident by chambering a round from the magazine several times. They discovered that this malfunction, firing when a round was chambered, could be duplicated at around 10-15 chamberings. They also found that at about 10-15 chamberings, many rounds did the opposite and the primers became desensitized enough that the rounds would not fire.
3) ……I went through my duty ammo and pulled a couple of rounds that were dimpled. I took one of those and loaded it a total of 13 times then fired it. I pressed the shot heard a click...Boom. The delay was quit short, less than a second, but noticeable. I have heard of this, slow burn, but never experienced it before.
That same block of instruction should include bullet setback, and probably update the 1965 incident to when it again became an issue 5 or 8 years ago in the army when it happened in front of a ranking military individual.
I harp on the point of bullet setback as it is almost never mentioned in cop courses, even in areas where guys use AR15s on a regular basis. I pull my primary duty mag awhile back and found 3 or 4 rounds which had the bullets noticeably pushed back into the casing. This is even with rotating rounds off the top.
The U.S Army has even put out guidance in this matter. The recommendation was to discard after 5 loadings.
Using the search feature would reveal several discussions about this.
https://www.m4carbine.net/showthread.php?33805-Primer-has-firing-pin-mark-in-unfired-round
https://www.m4carbine.net/showthread.php?58112-Re-chambering-an-unfired-round
https://www.m4carbine.net/showthread.php?183446-Are-floating-firing-pins-safe
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