Oh and I just remembered something else, Vogrimmler (I think that was his name), one of the engineers working on the StG45 rifle, went to France after the war and then on to Spain to help design the CETME rifles that were then redesigned into the G3 by H&K.
During his time in France he helped develop a carbine similar to the Stg44 and StG45 but it was designed to fire the .30Carbine round.
I know I have pics of the prototype somewhere and it's on the net somewhere, I just have to remember the rifles proper name but that way you could easily have an American StG44 type.
Edit: okay, I've been a bad boy at work and wasted company time looking for these...meh...
Anyway, the website I mentioned is here
http://www.forgottenweapons.com/
The French StG45 variant in .30Carbine is the CEAM Modèle 1950, a brief wiki page is here with one image but you can find others on the net with the rear stock folded and also one with the foreward grip (actually a bipod) folded down.
http://en.wikipedia.org/wiki/CEAM_Mod%C3%A8le_1950