This problem is with a custom PacNor .458 barreled smokeless muzzle loader. Bullets
have to be sized to have no resistance at the muzzle in order to push them down to
the tighter breach end of the barrel. Even at that, carbon builds to the point the
barrel has to be cleaned after very few shots in order to again get the bullet to the
powder charge.

Suggestions?

I believe that SOMEHOW the breach end needs to be lapped in a way that does not
enlarge the bore diameter of the muzzle. What method could one go about to do that?

If'n it were me, I'd be takin' this up with Pac-Nor.

Be careful, the last two times I sent barrels back to Pac-Nor for that same reason (centerfires tho) they would not stand behind them. I haven't bought one from them since.

I have had this problem before also. No purchase since 2005.

If no luck with Pac Nor then the lead slug lapping can work if done properly. It will remove more material from the tight spots than the loose. Read up on it or have a pro do it. It was common practice to do this on muzzle loaders to "freshen" the bore. This is why you see some odd ball calibers as the bore will increase slightly with each lap job.