If you insist on cutting it down....................
Here's my patented non-gunsmith home barrel cut down instructions:
To measure the barrel, put a cleaning rod or wooden dowel down the barrel with the gun's action CLOSED.
Mark the rod even with the muzzle.
Remove the rod and measure from the end to the mark. That's the actual barrel length.
To determine where to cut the barrel, measure from the end to NO LESS than 18 1/2" from the end.
Lay the rod along side the barrel with the first mark even with the muzzle, and mark the 18 1/2" mark on the barrel.
Measure the barrel to locate the cut off line.
Measure the barrel again.
Have a friend measure the barrel.
Measure it yourself one more time.
(Mistakes are not correctable, or forgiven by the BATF).
Wrap a piece of tape around the barrel at the cut line to use as a cutting guide.
Use a high-tension hacksaw frame with a new fine toothed blade.
Make a shallow two or three stroke cut in the barrel. (Make SURE you're cutting on the right side of the tape. See above about BATF).
After making a shallow cut, rotate the barrel and make another shallow cut, continuing to make shallow cuts and rotating the barrel until there's a shallow line all the way around the barrel.
Then just continue the process of making a couple of strokes and rotating the barrel until the barrel is cut through.
Lightly file the muzzle to remove cutting marks, then wrap some fine metal-type sand cloth over your thumb and use that to break the sharp inner muzzle edges.
Use a fine-cut file to remove the sharp outer edges of the muzzle, and touch up with cold blue.
The tape and shallow cut method prevents the saw cut from "drifting" and leaving an angled cut. This prevents having to try to square up the muzzle with files, since if you're careful, this will produce an almost perfectly square cut.
If you'd like a front bead, Brownell's sell a Remington-type bead on a small base that can be attached with soft solder or possibly the new Loctite Black Max adhesive. This too can be done at home.
http://www.brownells.com/aspx/ns/store/ProductDetail.aspx?p=5217&title=SHOTGUN+FRONT+SIGHT+BASE
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=6139&title=BLACK+MAX+ADHESIVE