I've shot 240gr FNGC Mt Baldy cast bullets in mine at a little less than 2000fps, 1" 100yd groups, same for 200gr Beartooths at 2140fps.
The SB2 10ga shotgun frame won't accept a rifle barrel properly, it's too big, as is the firing pin and hole. The frame sides are also much thinner than an SB2 rifle frame.
Tim