i have a colt h-bar 20" heavy barrel with a flat top reciever, and i would like to swap the 20" heavy barrel to a 16" lighter barrel. i've been told it's easier to just switch the whole upper assembly than to just switch the barrels, is this so? also, are there any legal factors i might need to be aware of? i want to keep the flat top, but i don't know how expensive it is to do the switch i want, or which would be the easiest / less costly route to do so. ultimately i want the light barrel in a flat top upper.