So as I was cleaning my Springfield M1A yesterday I noticed that it (seems) to be possible to fire the gun before the lugs lock into place.
I think it would be like a 1 in 50 million chance for it to happen but still. What I'm saying is that theoretically it seems to be possible to pull the trigger at just the right moment as to detonate the primer but before the lugs have locked without any sort of malfunction.
A M14 is a product improved Garand, the bolt and trigger mechanism are similar. John Garand designed a very good mechanism and incorporated features to prevent the hammer from contacting the firing pin before lug engagement.
You can see the function of the receiver bridge and hammer nose in this US Army 1943 Garand Training film:
Function of hammer nose and corresponding cam in rear of bolt explained @ 4:52.
Receiver bridge shown @ 13:10
With all the shooting experience Bart B has, without a doubt he has seen lots of “doubles” where trigger jobs failed during rapid fire. It was common for someone with a M1a or M1 to be shooting sitting or prone raped fire and have a “double”. In every incident I saw, the hammer followed the bolt down because sear engagement surfaces had been reduced to the point that the hammer was no longer being held back. Typically this was a trigger job that too much metal had been filed away. Hammer follow due to worn sear surfaces was a problem that did not just go away: if the hammer followed once, it would follow again on the alibi string, and we had the rifle removed from the line as it was too dangerous to be around.
Even though Garand designed a mechanical interlock to prevent out of battery slamfires due to hammer follow, I don’t trust the things 100%. They are not to be relied on, if it works, well you fell off the high wire and you were lucky a safety net was below you, but any rifle which the hammer follows due to mechanical wear must be fixed because, there is always the chance, even if I don’t see how, it could fail and the cartridge ignite out of battery.
Out of battery fire has been a particular concern of firearms designers, and all the good designs have features to mitigate the possibility. Of the firearms I own, the HK roller bolt actions are particularly good in preventing slamfires. The firing pin is positively blocked from forward movement until the action is in battery and furthermore, the firing pin spring is exceptionally stiff, preventing any inertial forward movement of the firing pin. The only way this action could fire out of battery is by some hard object lodged on the bolt face making contact with the primer.