Barrels for very small calibres and what's considered to be fairly low pressure shells only can be re-lined.
: Lining involves boring out the old rifling to a size that will allow insertion of the liner 'tube' that has the new bore and rifling inside. The barrels would have to be very large to do this and I don't think it would be practical in a BPE at all.
: Some coatings, like Danzak and Moly would invariably help with minor pitting to obtain good accuracy and clean shooting from a piece that isn't in too bad shape.
: A gentle polish with somthing like FLITZ or SOLVOL automotive polish will help about any slightly rough to bad barre. Even lighter is JB compound, put on a cloth patch and run up and down the bore with bore solvent, will bring up a nice shine if the bore isn't bad at all. Rougher bores need polishing/lapping with compounds, not out of the picture for the home hobbiest.
: Picture the sewer pipe modle 1871 Mauser I just did. The obre now shines, shows some minor pitting, but was dull, black and looking like vocanic rock when I got it. It doens't even lead now, with full power loads using either Lyman's BP lube with Black Powder or Lyman's Moly lube with smokeless powder (both stick lubes for lube sizers).