I have personallyintentionally polished the bore of a muzzleloader. If you think about it.. when you JB Bore Paste them you are technically doing that, so yes I have. A lot of times a rifle, old or new, can have burrs or ruff spots in them.. Some mild abrasive like JB Bore Paste or some use a jewelry paste would have the effect of smoothing out the ruff spots. So I think a lot of us do.

As to whether it helps break in a barrel faster... without doubt. I have seen rifles do a day and night change over on the range with a simply application of bore paste. Especially in the traditional roundball barrels. It used to be said, never get too excited about your muzzleloaders accuracy until you have a couple hundred rounds through it. That was because many of the old barrels, that were rifled were done in such a manner that the bore was ruff in spots, had burrs, etc.. and it used to take 200 rounds to smooth them out. The new barrels I personally feel are much better as far as the ruff spots in them.