Msp is correct. The 10ga can only be fitted to the larger receiver...same receiver as the 12USH.
Both the 12ga and 20ga TDC have choke tube barrels, so either one would work. Also, if a 20ga will work for you, there is the 22" choke tube barrel that comes on the Pardner Synthetic Youth model. That's only 22" long. Best choice IMHO...24" Turkey barrel.
cookiemann