Beretta now has a 'baby frame' for 28 gauge/.410. They also sell 20/28 combo sets, plus the above mentioned small gauge barrels that fit 12 gauge frames. Easy to suppose that the 'baby frame' takes different barrels than the 20 gauge frame, but I do not know that.
Cole's advertises a list of extra barrels to install on Beretta O/U's. They always fit the barrels to your gun, so your old barrels still work, as I get it. Ejector timing may be part of the fitting. I would email them at Cole Gunsmithing [rcole@colegun.com] questions, and peek at
http://www.colegun.com/barrels.php to see what is listed. The FAQ answers some questions, too.
