The Halo 2 and Halo 3 SMG's are the same weapon, but have slightly improved graphics. What you are asking for, I believe, it just the retracted version of the SMG. I suggest that you use one of the SMG files supplied on the Halo Costuming Wiki, and simply cut out the area of the stock that would slide into the butt of the rifle. Then, glue the butt of the SMG to the rest of the SMG. That will create what you want.