Well, I slugged the barrel, and found out the following..
The groove diameter is .464. The land diameter is .448.
When a slug is pushed all the way through the barrel, the measurements are the same.
The rod and jag with which I first noticed the problem, is almost the same diameter as the bore. I don't feel much difference when I use a smaller rod. I also don't feel much if any resistance when I push a slug through the middle of the barrel.
What's going on? I can't see anything wrong when I look through the barrel. Is it possible that the bore is not straight? It is centered on the muzzle and breech.
If the bore wandered off in straightness, it would explain why the rod jams.