Re: How common are "slamfires"?
If we can leave off the anti-BATFE rants, let's get back to real slamfires. I have never had one with any rifle or handgun (except SMGs where it is supposed to happen).
Many people confuse doubling caused by a defective sear or hammer with a slamfire. A true slamfire occurs when the rifle fires as the bolt is closing, without the hammer being released, and will almost always result in firing unlocked. That again almost always results in destruction of the rifle and very often injury to the shooter. When someone tells me (as one chap did) that he had five "slamfires" in an afternoon, I know he didn't have a slamfire.
It is common for an M1/M14 rifle to show a slight dimple on the primers of rounds that have been chambered; due to the way an M1/M14 works, that is common, but also of no concern. But people see that, and inevitably think one step beyond, feeling that if there is a light dent, there will have to be a heavier dent sometime. Not true, and not a problem. Barrng tampering or out-of-spec parts, it just doesn't happen.
And if it does, you won't have to ask if you had a slamfire. You will know. Holding your bleeding head and picking brass out of your eyes is the first clue.