CW, you might be confused by my 10ga thread, the 12ga USH uses a 10ga frame which is a heat treated SB2 frame, but not the same as an SB2 rifle frame, the chamber swell is 1.2" on the 10ga and 12ga USH, so it's too big to fit a 1.1" rifle barrel properly. That and the fact that the shotgun has a larger firing pin and recess, plus the side rails of the frame are much thinner than an SB2 rifle frame make it unsuitable for any rifle barrel and the reason why H&R won't fit them to it.
Sorry about the confusion.

Tim