My dad has a wilson combat barrelled ar-15, ever since he had it, he will drop a shell in the chamber and hit the release, then he will put the mag in. We'll this time the ar-15 fired when it slammed home. Luckily he had it pointed down into the ground. The casing on the left is from the shot. Im guessing maybe soft primer and the floating firing pin along with the bolt not being slowed down by the magazine. He has done this since he had this rifle going in two hears, but has had several and has done the same. I've heard of ar-15 doing this but have never actually knew someone it happened to. Pnw factory ammo. Are my thoughts in line on what happened.