Getting 18.5 Inches
Next, you need to know how the boys in blue measure barrels so you can be compliant. They close the bolt. Then they drop a dowel marked at 18 inches into the muzzle. If their 18 inch mark ends up outside the muzzle, a trip to the pokey is in order. So.....how do you achieve this nirvana, the shortest legal length without going under?
First, settle on 18.5 inches to give you a little room for error. Get a dowel that will drop into a shotgun barrel. I use a 3/8 inch dowel. Next, mark 18.5 inches on your dowel. Close the bolt of your gun. Drop in the dowel, 18.5 inch measurement first. Now, mark the dowel again at the end of the muzzle. This is your present barrel length. Let's suppose for the moment that is 28 inches.
Now remove the dowel, and subtract the 18.5 from 28, ie the least legal length from the present length. Our result is 9.5 inches. Now, measure back from the muzzle 9.5 inches and mark the measurement on the barrel. Check it again. Put everything down, go have a cup of coffee and check it a third time. When you are totally satisfied that you are of legal length, make your cut.
I like to use a pipe cutter, but a chop saw could also be used. The key is to get a 90 degree cut. Many barrels have a taper, and that can throw you off on a chop saw table unless you compensate for it. Once your cut is made, remove the burred metal, burnish the area of the muzzle, and cold blue it. Finally, drill and tap a hole for a bead. I prefer a big brass bead.
Congratulations! You have now lowered the monetary value of your shotgun, but increased it's versatility markedly as a defensive weapon.
A couple of considerations:
1. The overall length of the shotgun must remain over 26 inches. If not, bad legal juju. As long as you have a full stock, no worries here. Be careful if you decide to make your own pistol grip.
2. Many magazine extensions need more than 18.5 inches of barrel for support. If you are going to use a magazine extension, install it first!