Re: How common are "slamfires"?
I've owned an M1A for many years, and yesterday experienced the first episode of either a double or a slam-fire. I was shooting military ammo, with crimped primers, FMJ bullets, of uncertain origin. Primers are coated at the edges with black sealer, as are the shell cases at their mouths.
After firing about 20 rounds, the rifle doubled, the second report being very close to the first, while the trigger was still being pulled. There was no gas escape or other visible abnormality. I noted that the tip of the firing pin was protruding from the bolt, but it easily retracted when touched.
I collected all the spent cases and every primer appeared to have an identical pattern, with a deep central impression. None of the cases were bulged or deformed. After checking the action and the bore, I fired the rifle several additional times without further incident.
My impression is that the rifle doubled, as you describe, rather than a slamfire. I'm presently giving it a good takedown and inspection for any signs of wear, which is unlikely -- it has probably had no more than 1000 rounds thru it. The rifle is completely stock, no modifications or add-ons.