Probably because I never had to eject a chambered round in 28 years of USAR duty, (yeah, I had it easy :-) I was surprised to see that the primer of each unspent round (the last one I put into a mag) that had been chambered and then ejected in order to cease fire and clear the range, had a small dent, not unlike a fired round.  I reloaded each of the 3 cartridges that I had ejected and all subsequently  fired fine.  But is this a dangerous situation- can a round actually go off when I merely release the bolt catch and allow it to fly forward and chamber a round?  What causes this?

It's normal, and as long as everything is in good working order, its not a problem.

Unlike many other firearms, the AR15/M16 does not have a rebound spring on the firing pin. When the bolt is released and a round is chambered, the firing pin has inertia built up that causes it to lightly strike the chambered round when the bolt comes to a stop.

Nothing to worry about.

Same info as neilfj just gave you, but for  good pictures of this, read the answer to the question "Q. I chambered a round in my AR and then unloaded it later.  The primer has a small dent in it, apparently from the firing pin.  Should I be worried about this? . . ." under http://www.ammo-oracle.com/lcugly.    

[url]www.ammo-oracle.com[/url]

see question above there.....

Mike

The M1 Garand and SKS do the same.  No worries.