I know an idiot that did something like that! Alright it was me.. I put a carrier in without a firing pin and the cam pin turned and got stuck. I eventually worked a firing pin back inside of the bolt and got the bolt carrier out. If you can work a firing pin into the carrier and its still stuck then its the stuck chamber guage. The cam pin marked up the edges of the relief cut in the aluminum receiver from me trying to pull the bolt carrier out before I got the firing pin back in. Its been many years and I don't remember how much the bolt carrier would come back before it would stop though.
EDIT: I played around with spare upper I have and if you have no luck getting a firing pin worked back in there is another option. This seems sort of familiar. The bolt will turn easily witht the cam pin turned 90 degrees the wrong way if you partially pull the barrel out so that the bolt will start to turn before it normally would which is the clearance you need so that the longer side of the cam pin has enough clearance to get past the cutout in the upper. If that makes sense. Hopefully one of the military guys will have a quick and easy method of getting the cam pin turned correctly.