First of all,
Put your hands together and thank the Lord you were not seriously injured.
Looks to me from the photos that the case ruptured when outside of the chamber, perhaps that is what scraped the metal on the inside of the reciever. These are my guesses:
Maybe the firing pin stuck and "slam fired" the round, which fired before fully entering the chamber, thus accounting for the floorplate poping off and the reciever damage. Check the bolt carrier and firing pin, as well as the springs, for any wear, damage, obstructions, etc.
OR
The recoil spring jammed up, got caught somehow, etc.(ie: the sound you heard) and the lack of forward pressure on the bolt caused it to open prematurely. The cartridge exited the chamber before the bullet, thus the case was subjected to the full pressure that existed in the barrel. The result is that the partially unsupported case ruptured, which could also blow off the mag floorplate.
I don't claim to be an expert, but I hope this helps you out. PLEASE be VERY careful when shooting this and you probably should have this checked out by a Bushmaster armorer/gunsmith, or at least someone who is very familiar with that type of action.
Just my .02,
Urban
[sniper]