I recently acquired a used 50 cal T/C Pennsylvania Hunter 1:66 twist barrel with a rusty/pitted bore. After an extensive cleaning process, I was able to get the bore to shine once again with some pitting left over, mostly in the grooves. I'd guess the bore at 80%.
Not expecting anything great, I sighted it in for 25 yards. After that I was able to shot 12 patched balls through a one 1" diameter hole at 25 yards. I was surprised at the accuracy considering the crap that I got out of the barrel. The only negative seems to be it needs cleaning more often than a "pit-less" barrel.
How much pitting is too much?