There are a lot of things about H&R that don't make sense, but you cannot put any barrel on the 12 gauge USH frame besides that 12 gauge USH barrel or a 10 gauge shotgun barrel. This is due to the large barrel used for both barrels. Also the frame is an SB2 shotgun frame, which is different than an SB2 rifle frame.
For what it is worth, you can put low pressure barrels like 357 mag and 44 mag on SB1 shotgun frames. You can also put any shotgun barrel except for 12 gauge USH barrel or a 10 gauge on any SB2 rifle frame.
Daman