shorten shotgun bb l

A link really isn't needed. Take a hacksaw and cut it off where you want it (I wouldn't go below 18.5", minimum legal length is 18"), and dress the end with a file or get this tool from Brownells to dress it.
http://www.brownells.com/.aspx/pid=5171/Product/SHOTGUN-BARREL-FACER

Don't forget to get the pilot bushing with the reamer if you go that route.

I face the barrel off with a vertical mounted disc sander with a jig for angle. I use a fine round file to barely take the sharp edges off and then use 800 grit around a finger to barely roll both outside and inside edge. A little cold blue and you're done.

If you want a front bead sight on the cut-off barrel?

Mark the new location for the hole with a center punch while you still have the old bead on there for a referance point.
Just wrap a string around it, pull both ends tight centered on the receiver, and mark it.

Once you cut it off, the job of finding center gets much harder.

rc

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This advise is Brilliantly simple for what could make for a complex problem.
rc>smarter than your average Bear