Just got it back today. Patriot Shooting Sports was amazing. Super professional, did tons of leg work and got it back to me in perfect shape. It took a while, but that was because of Bushmaster taking their sweet time on parts.
Here's what happened: Bad Ammo.
I pulled the trigger. Pop. Boom. Face full of powder and a non-functional gun. The shell was stuck in the chamber. It came out easily enough with a rod, but the extractor was broken, and the aforementioned magazine was trashed.
They determined this:
A reloaded round had a bad or loose primer pocket (it appeared to be oval, maybe damaged during swaging?)
When the trigger was pulled the primer popped out of the head, but managed to light off the powder.
The lit round went off like a bottle rocket forcing the case forward into the lands, bending the mouth.
It still managed to get enough pressure to get the bullet out of the barrel, but a lot of the pressure went backwards, melting the primer and blowing apart the magazine.
I'll be turning in a claim to the manufacturer of the ammo.