Is it possible that some of the M&P15s have more recoil than others?

I came home the other day with bruises on my shoulder/peck area. That evening I saw them in the mirror and could not for the life of me figure out what had caused them - it didn't occur to me that the recoil was THAT bad (besides, I had done other yard work that day and at first figured I must have done something in the yard). I keep a pretty firm grip on my rifle and pull it in to my body. Could that be the problem?

Didn't mean to hijack the thread, but I hate to admit that I have some idea what the OP is talking about.

Could some of these units be WAY over-gassed?