I've had a Mega lower built & ready for the carbine upper to be assembled after the DAY. I just couldn't wait. I bought a complete 20" Oly post ban upper that snuck in under the ebay police radar. Slapped it together & went to the backyard range to give it a whirl. The mag seated fine & the first five rounds worked perfectly. I handed the rifle to my son's friend Mike, age 17, to give it a try.....grin factor +12 He'd never fired an AR before...hooked another one The kid did great, five shots @ 100yd, he hit 4 steel targets & one plastic milk jug firing off hand.
I reloaded the mag & went about a little more trigger pullin'. First round fired and then it jammed WTF?? Initial field exam was that perhaps two rounds stripped out of the mag & stuck in the chamber.
After disassembly back on the bench I discovered a ruptured case...whew. No KB. No sign of damage.
It was a reloaded case with a NATO mark on the base. I'll list that failure as "shithappens"