ALL buttstocks are interchangeable, an you would be correct about the differences in forend barrel channels. The 10ga and 12ga USH forestocks use the widest channel, followed by the 20ga USH. Most of the others are interchangeable. Some of the Ultra-Lites, Sportsters, and Versa-Paks have smaller channels. For instance, the forend off my Sportster .17 HMR bull barrel will not fit my .223 Ultra, Toppers/Pardners, or Huntsman due to the channel being to small. The forend off my 20ga USH will fit the smaller barrels, but leaves a gap between the sides and the barrel. The Sidekick forend is only made to fit the Sidekick, due to the shorter frame.
The exterior dimensions of all of the frames (excluding the Sidekick) is the same, the 10ga and 12ga USH frame just has more material removed from the inside walls to accomadate the larger barrel diameter. Although all buttstocks are interchangeable, a Monte-Carlo style comb will probably be to high for most open sights, and way to high for a bead sight.
