Probably wasn't a slamfire from OP's description of the event. There was a passage of time from the round chambering to the discharge, only after he had aimed downrange and flipped the safety off. A slamfire would have been instantaneous when the bolt closed into battery. He didn't mention loading from a magazine, but single loading an AR is not really easy; if all you want is a single shot, it's much easier to load one round into a magazine and load the gun from there.

I shoot a M1A quite a bit, and they have somewhat of a reputation for being slamfire prone if single loaded, or if primers aren't fully seated in the case. Although I've heard tales from some shooters, I've never experienced one myself, even though I have single loaded many times. I just never drop the bolt from the locked open position.