Thanks for all of the replies. It is reassuring to hear that it sounds most likely to be due to the ammo. The first fifty rounds that I shot that day were what the gun is normally loaded with, Federal 124gr JHP +P+, and it shot perfect as usual. I mainly wanted to test the new mags that I had just gotten (thanks Natchez!) to be sure that they worked ok. I had half a box of the UMC stuff so I took it along. Next thing I know the gun was stuck. I was shocked to have the gun get stuck like that. Jamming up is one thing, but getting completely stuck is very disconcerting.
To follow up what people suggested or asked:
Stock barrel, the gun is entirely stock.
I inspected the recommended parts and they look fine. None of the signs of failure like described.
Though they don't sound like the cause, new springs probably would be a good idea.
It wasn't the last round in the mag. There was probably 10 left in it.
I guess my only option now is to take it back out shooting and make sure it works.
hanksArmin