The barrel doesn't appear to be one proofed following the Massachusetts law, as marking would be P over inspector initials, then next to that M over date. But is it an old military barrel? I don't see an eagle head, but there is P over US. Moller says many New England Militia muskets were made from surplus government barrels. In that case, since already proofed, they did not have to go through Massachusetts proofing. And of course it may have been made somewhere besides Massachusetts.