20's or 12's for the US & Canada - 16's not very popular here. Winchester barreled and chambered a few Model 12's in 16 bore & many Euro hunters living in Canada have Euro 16's in SXS 16's, drillings with 16's and 7mm or 8mm rifle barrel, or even 9.3's for the rifled barrel as well as over/under combination guns. A friend of mine has a 2 barrel gun (Merk or Kreig) with choked 16 bore smooth over a 7x65R. The lower rifle barrel is extremely accurate (1/2" for 3-shot groups @ 100 meters) & makes this a most useful rifle for hunting here - smooth for grouse and rifle for moose, deer, bear or elk.