He probably meant .410 gauge shotgun, not .41 magnum revolver. Armory had .410 gauge at 2(9 again),-10/-20/-30, -1 str, flip in capacity for the specific weapon you want (probably 1).

Update to GMC would probably be basically the same thing as a low-caliber shotgun, 2(9 again),10/20/30 -2 str (str values are usually slightly higher. Dump it to 1 if you want). Size 2-3 depending on cane's size, init -2 or 3. I don't think there is any set rule for conversion, so mess around with the numbers however you like!

I'm mainly a hunter guy, and can't give a blanket answer on the...