If that's the 12ga ultra slug heavy barrel (10ga barrel with a 12ga hole through it), you won't be fitting a 20ga or any other non-10ga barrel to that frame. Says "SB2" on it, but it came from the factory with the slug barrel, not a rifle barrel, right?
I wish they'd named the 10ga frame something distinctive, instead of giving it the same "SB2" name as the rifle frame.