Typically when I chamber a round in any gun other than a hunting rifle or carry gun, it gets fired. So I was caught off guard when I cleared my m&p 15 at the range for cease fire and noticed that the primer had been struck (lightly). Out of curiosity, I ran about 5 live rounds into and out of battery and all were dimpled. I have an ar but am far from an "ar guy". I know the firing pin floats freely but my gut tells me that this is not supposed to happen. Any input is welcome. Thanks.