Look at your barrels, the bore diameter is stamped near the mono block. With the muzzle pointed to your left you should find the information near the mono block on the upper barrel of an O/U and of course the lower barrel of an un-single. My barrels read "18.6 12/70". My TS barrels (2006 manufacture) are stamped 18.6 mm or .732, my trap combo (1999 manufacture)is stamped 18.7 or .736. The trap combo is a fixed choke gun.