"You can't get BB shot in a .28 gauge shell.
Not commercially. You could load them yourself, though. You could even do buckshot, although I can't imagine why you'd want to.
Commercial 28 gauge shot shells are limited pretty much to 7 1/2, 8, and 9 shot. The local Fleet Farm here only has 8.
I have 6 through 9 for mine. I've never seen slugs or steel available for 28. And I have looked.....