Pitted bores, more often than not, are (or, more accurately, WERE) the result of corrosive primers, which haven't been used for some time.
There have been many guns that have experienced pitted bores since corrosive primers went the way of the washboard. The causes were/are buildup in the bores of lead or plastic. With plastic shot cups used in most commercial loads today leading of the bores is mostly a non-issue. But, after corrosive primers, and before shot cups, it was a major cause of pitting. Today, plastic buildup in bores contributes to the same thing as leading used to. What happened/happens is, the lead or plastic residue builds up in the bore and moisture collects underneath. The moisture then causes rust which causes the pits.
Chrome lined bores are a blessing, IMO. I love them. As the Jewish mother said, "What's not to like?"
SRH
