All of the LeFechaux action Husqvarna hammer doubles have rather thick barrel walls. My 12bore M15 has nearly pristine bores and did have them when I bought it (it has been heavily used with BP, Pyrodex and H777 loads for about a decade). A M20 and a M17 have rather pitted bores, as does a very heavily used (by us) M51. I do generally stick to sub 10K psi loads with 24 or 28 grams of shot.
Neither clays or feathered birds or patterning boards ever seem to become aware they were shot by pitted bores.
All I have ever done to these bores was to "smooth" them with 400 or 600 grit oil-wet emery paper, making it easier to scrub crud from the pits and to remove as much hard crud and rust as possible.
SOP for these guns is to spray some Rem Oil in each bore and stand barrels in corner and let oil coat entire bores. This does raise a fair amount of crud from pits and, I presume, penetrates into the pitts. At least, I never have been able to notice additional pitting during the many years I have used.
One can always become a "pitt snob", claiming that those pitts retard the wads just enough to give superior patterns.
Niklas
