There's certain modes of failure in the AR15 that make me cringe, and a stuck cartridge/locked bolt is top on that list.
I had a friend at the range get a stuck cartridge in his chamber once. He brought me his rifle and I said "dude, does this have a loaded cartridge currently in the chamber?"
"I don't know."
{FACEPALM}
Greeeeeeat. He had been firing lacquered steel case for god knows how long, and switched to brass without cleaning. Lacquer residue glued his first brass round solidly in place. Bolt wouldn't budge. Period.
I had a rubber mallet in my range bag, and told him "If I bust your charging handle, I'm sorry."
It took about 50 moderately strong whacks before the vibration and shock finally got things moving. Didn't break anything in the process, fortunately. (Was afraid of bending / breaking the charging handle; did that once before.)
So, neck size if you want! But be prepared to bust parts getting it opened up if it don't work out.