Just checked out the Shotgun Box website; they have some interesting adaptors shown there. How about .45ACP or .45Colt in a 577/450 Martini-Henry? Or .32ACP in a .303Brit?
Gotta be careful when ordering, many of these adaptors are offered in both rifled and smoothbore versions, make sure you get the type you want.
I am looking forward to trying .357Mag and 9mm in some of my 12-gauges. A SxS or O/U double with a .357Mag in one barrel and a 12gauge (or even .410 with another adaptor) in the other barrel, equipped with open sights on the rib, could be a fun and very utilitarian "backyard pest" gun. And I have a few boxes of .22 birdshot loads that might be fun out of a 12-gauge barrel with a .22 smoothbore adaptor. I already have doubles that I have modified with rifle sights for slug use....hmmm...
I've always wanted to shoot House Sparrows and Starlings with a dedicated Cape Gun...![]()