Other then the ruff spots (if any and I doubt that since you have such a quality rifle) smoothing out, a barrel does not really break in as in wear down all that much except after a large amount of projectiles shot. I have yet to wear a black powder barrel out. At least not what I noticed. I think it is more of a combination of the shooter learning the barrel. They learn the feel of the slide of the sabot and how to swab the barrel in a more efficient manner. They just get used to it is what I am trying to say, and it all becomes easier. Although the right combination of things can sure make a difference.

Now you can "wear" a barrel down through a process of lapping but that is a thing I reserve for a rifle that will not shoot anymore due to a number of different reasons. You could take some J-B Bore Paste and a chunk of scotch brite scouring pad on a patch worm and work the barrel over with that. It is a great way of taking out the ruff spots and cleaning the grooves. I use it because if there is any lead, copper, or plastic fowling in the barrel it will remove that as well.

I think once you find the right sabot projectile combination you will be good to go.. Good luck with the rifle.