Sentryau2
Are all semi auto rifles capable of doing this? Honestly I'm hoping after I go to sleep I'll wake up and this will all just be a bad dream.
"This," meaning denting the primer when the bolt closes.
Yes, many but not all. For example, the M1 Carbine does this, as does the M1 Garand, as well as the SKS semi rifle among others. The SKS is fairly well known to slamfire with its floating firing pin, to the point where at least one guy in TX* sells replacement firing pins for it which are either lightweight titanium or sprung backward, or both, depending on the exact model of SKS.
The firing pin can also be stuck forward ("protruding firing pin") from dirt and cause slamfires. In fact, it can cause an unexpected mag dump.
With the SKS, the original design had a sprung firing pin, which was later modified to not have the spring in it. (I note that most combloc ammo has somewhat harder primer cups than American primers. Hence slamfires and mag dumps were not a common problem until folks started using American "softer" primers in them.)
With the M1s (both), it was deemed not necessary to have the firing pin sprung, and it was known to be a slight danger but only in extraordinary circumstances. One such extraordinary circumstance is re-chambering a cartridge which had been previously chambered once or twice --which ordinarily does not happen in combat, especially with the Garand's
en bloc loading system. **
And yes, denting the primer does sensitize it somewhat. In fact, the act of pressing the primer into the primer pocket during manufacture or reloading the cartridge forces the anvil into the priming compound
slightly and sensitizes it. One can verify this by examining an un-re-loaded primer and noting the anvil sticks out the bottom of the primer cup a matter of a few thousandths. It is seated flush to the bottom of the primer cup in the act of pressing it into the primer pocket.
It is possible in Sentryau2's case that the bolt release button got bumped on the way down to release the bolt. Thus, the rifle could have actually fired in so-to-speak "mid-air" on the way down. (At the range, I put a mag in with the bolt locked back and close the bolt with a rap of the heel of my left hand to chamber the first round.)
As was mentioned, "I hate floating firing pins." Well, "dislike," anyway.
Your incident was indeed an example of a "perfect storm" of events. I.e., "extraordinary circumstances."
Terry, 230RN
*
http://www.murraysguns.com/sksown.htm
There is an interesting and informative discussion of this whole matter in that website.
**That's why, in testing a newly-acquired semiauto, I sometimes put a dummy in the mag, and then only two live cartridges in for the first couple of tries. Depends on the gun. Another cause of double-tapping or mag dumps might be a dirty or damaged disconnector mechanism. I bought a 1911, fairly new condition, which double-tapped, the cause being traced to burrs on the disconnector. A bit of polishing on the disconnector cured that problem.