All excellent answers, but I have never had good results shooting 2 3/4" or 3" Foster slugs in the 3 1/2" chamber. We are testing a new load for the 3 1/2" smoothbores (and rifled barrels) now called the Tusker Plus (in the 3 1/2" hull).....but, since it is a hard cast heattreated .727" - 600 gr slug, it has to be shot in a cylinder barrel or a .727" choke.
Regards, James