I was also thinking of an undercut to hold it in place.
The pits might be pits as caused by a corrosive, as in primer or in the powder itself.
If so, the pits might just be undercut already, like chlorate and perclorate fouling does to some steels, rather than simple dished-shaped pits caused by normal rust/oxidation.
Either way, they must be thoroughly cleaned. In the chlorate damage, there might already be the fill-holding undercut.
Getting a good look with a Hawkeye bore scope should show the nature of the pits.