Just measured my 410 Skeet-er , marked C and O. Both barrels had .010" constriction. Guess that's not much help. By the way, we had often "guessed" that the C and O stood for Coming and Outgoing.

My 28 gauge Skeet-er is marked I and M, with the constrictions of .010" and .015"

Last edited by Daryl Hallquist; 09/15/19 10:28 AM.