Well most tubes, well all screw into the end of the barrel. The choke controls the constriction on the shot as it exits the barrel. It either tightens it up, or lets it spread faster, or open up. Depending on each hunter, you want a tighter pattern that delivers most of the payload in a 10 inch circle so you are putting the shot on target, the turkeys head and IMO at least 12 or more pellets consistently in the vital areas of the turkeys head! [8D]