So I know what happens when you put .300BLK in a 5.56 barrel (not my gun by the way) but what would happen if you shot 5.56 through a .300blk barrel? (Disclaimer: I am not going to try this nor do I want to try it I'm just curious)

Ouch

223 in 300 should not cause any catastrophic damage...allthought it may cause rifling damage as it bounces down the barrel.

I am trying to figure out how the primer was struck on the .300 blk. Obviously the .30 didnt chamber in a 5.56 barrel so I am guessing the floating firing pin or lugs somehow ignited the primer.

If you repeatablily chamber a 300 blk out in a 223 AR, it can set the bullet back far enough to chamber, if the load used is not a compressed load. Basically, the ogive of the .308 bullet will set on the shoulder area of the 223 Rem chamber. The blk out should not have enough room to chamber, unless you force it in, and fire it. But I have heard guys being able to get the bolt closed on a round. I run a compressed load on all my supersonic rounds, so bullet set back would be very difficult to achieve. A sub load, maybe.

Better question....is the guy alright? That is right infront of the face.

You shouldn't have any issues firing the .556 in the .300 barrel. This type of process is used to fire form brass on a regular basis for many wildcat chamberings. I wouldn't expect it to cycle well though as you may not have adequate back pressure.

It was actually the guy's wife shooting at the time and she grabbed the wrong mag...apparently took some shrapnel in the arm but will be ok.

Ouchie!

yea that'll happen, same thing happened to this one

My wife would never come back.