I've shot BOSS 3's and 4:s out of my Browning Superposed...it's an older, fixed choke gun (mod/full, 3" mag 12-gauge). I thought they were very usable - if I do my part, the shells do theirs just fine. The issue I had one day was just not shooting well, which is an expensive proposition with BOSS. I know some people have had issues with bismuth shattering inside the birds and leaving pellet pieces all throughout the meat, but I haven't done any autopsies to figure out if I'm getting anything like that.
I used to handload ITX for another couple of old shotguns I have, but they quit making ITX, so now it's buy BOSS, handload bismuth, or only shoot the guns with lead on clay pigeons or doves.