Is it possible the barrel is “shot out” after 50 years?
Definitely not. I bought this in 1984 it was only 11 years old and seemingly unused then. I hunted with it but never exclusively using other rifles also depending on the circumstance. This has been my brush gun and I have taken 2 deer with it but wouldn't hunt with it now since it went bad. I doubt it has 2500 rounds through it and if that much. No Winchester is going to wear out with that kind of round count on it.
I've been reviewing what Jericho said and I think I'm going to do a breech plug bore soak just in case the throat is heavily fouled and I can't see it.