Somebody sent me a tang-safety 77 years ago, saying he could not get it to shoot. I couldn't either, and went through the entire rifle, never finding anything wrong--until I slugged the bore. The TIGHT spots were .279 in diameter, and there were several much looser spots.
Many of the barrels on the tanger 77s were very good--but a few sucked.
MD is dead on correct.
To fix that you have to lap out the tight spots so that the slug slides nicely and gets a tad bit tighter as it heads to the muzzle end.
If it loosens up as it’s heading out = you are screwed and it won’t shoot...
Tight spots are easier to deal with than a single loose spot, much because of how you pour the lead slug
**. - BUT before you touch it lapping (and you can ruin a barrel by doing it wrong), make sure you have all the fouling out of it so you aren’t getting a false read.
Most people don’t have the right equipment to slug a bore so becareful not to damage the bore trying to slug it.
