I don't believe it was a "slamfire". It sounds like a double fire. I can get my M1a's and Garand to do that if I'm really light on the trigger. It seems to help prevent this if I seat the rifle snug into the shoulder and get just a tad aggressive on the trigger pull. If I squeeze lightly on the trigger till it breaks, I can almost make it happen. A "slam fire" in the M1a/M1's generally exhibit more critical results, such as visible damage/destruction on the weapon/shooter.