SB does not stand for small bore.
Rifles are denoted as SB2 receivers, which are stronger than the SB1 recievers used in shotguns & muzzleloaders. A similar low strength receiver for the rimfires is also called SB1. The larger frame for the 10ga shotgun is called SB2, but is not interchangeable with the others.
Any non-10ga SB2 receiver can be fitted with a .223 barrel. The choices include a 20" ultralight, 22" standard weight (quite heavy) and 22" & 24" bull barrels (very heavy). I'd like to see the 26" standard weight barrel made available in .223 or 22-250.