C3 should be phosphated and painted ala Britishpractice.
The reason for the confusion is that there were multiple barrel lengths and twists available.
C3/M82 should be 26" 1/12", or 26" 1/14" (depending on which CFTO you read)..."the heaviest contour available"
C3A1 could be 26", 24", 22" and 1/12" or 1/10" the standard should be 24" or 26" 1/10 (or 1/12 depending on which CFTO you read)...
consider it totally F'd...