There is no physical or technical reason you can't put a short barrel on a TC with a rifle stock. Unfortunately, there is a legal reason. You could find yourself in a world of legal trouble for doing so.
Obviously, a 223 rifle with a 10" barrel is nothing but a machine for slaughtering humans, whereas a 223 rifle with an 18" barrel is harmless, as is a 223 handgun with a 10" barrel.
