I have two M21's bored SK1 & SK2. Both 12 bores and both made in 1935. Barrels are original and untouched. One of the guns mikes out at .006 for SK1 and .010 for SK2. The other one mikes out at .009 and .016. I am somewhat skeptical that the reverse bell on the SK1 chokes has any measurable effect on anything. At any rate both of these guns perform superbly out to 35 yards with appropriate loads and the second one is deadly to 40 yards with the left barrel at .016 constriction.