A standard tubing cutter...available anywhere plumbing supplies are sold...does an excellent and squared off job of shortening the barrel. I usually use a Dremel to smooth the edges.

Good advice from curt on how to measure. A choke tube would be prohibitively expensive, IMO, and there is no guarantee your barrel wiould be thick enough to take one in any case. You will need to drill and tap a hole for a new front sight, or you can paint a wide white line down the center of the barrel that will work in a crude manner. Once you whack the barrel, Cylinder bore (NO choke) is what you have.