cjclemens, thank for the correction on the difference between the 5.56 and the .223. I have never noticed a significant accuracy loss shooting .223 in my 5.56 AR, but I've heard that it can occur.
I don't know everything about primer ignition, but I do know that velocity has something to do with it. As a test to see how safe carrying a decocked Mosin-Nagant with a round in the chamber would be, I chambered a primered case and commenced to hit the back of the cocking piece with a rubber hammer. Never went off. I had a nicely dented primer (not as much as a fired round) but no ignition. This is with the firing pin sitting on the primer, not a floating pin. I still wouldn't carry it that way, but it shows how much velocity is required to ignite a primer. I don't think an AR-15 bolt can create enough force and velocity to ignite the primer. I guess it can happen, but it doesn't seem likely.
I've never had a slam fire in neither my AR nor my AK. I haven't shot an SKS enough to know about it.