Mr. Brown, you're right. I bought a pair of new 20 guage 26" barrels from the old Numrich's when they were closing them out for about $50, and fitted them to the frame of a 16ga 28" that I had. This was not an experiment that worked--as I recall the short 20ga tubes made the rather well-handling 16 "feel funny". I didn't like them and shortly found someone who thought the combination or "set" was cooler than I did and got all my $$ back, and then some. Since then I have a friend in upstate NY who has a 26" 20ga M24 that he has been using very effectively on grouse/woodcock/cottontails/jumped ducks for years, but I think it wouldn't work for me.
I don't really recall the chokes on those 26" 20ga barrels. I always believed they were the conventional IC/M, but couldn't prove it now. The 28" 16ga tubes were a rather UNconventional IC/M; that I DO remember.
