Not gonna work very well on the 12ga at all, the barrel is 1.20", checkered syn forend is 1.30" wide, wood forends use a different spacer on the 12ga USH, there's no spacer on the synthetic forend, so the ears that fit against the frame are going to be real thin, same for the sides of the forend after it's hogged out. Dunno about the 20ga USH, don't have one, if it uses the regular spacer, it should work.
Tim