A barrel can be manufactured shorter than 18.5. You can't do it at home. You can cut down a barrel to 18 (+)" as long as the OA is 26+".
I cut down an old single shot to 18.25" and an OA length of 28 by shortening the butt. I used a hacksaw and set of files to clean up the cut and make it plumb. I cut the stock with my chop saw so it was nice and straight then re-attached the butt plate and filed it off to match the new smaller profile. I painted all the individual pieces with matt black wood stove paint. Attached a set of sling swivels and a shell holder sleeve on the butt now it's my "walking gun" I use it when I'm crashing around out in the bush. Oh yeah, I didn't bother with a bead either, no point really, I'm not going to snipe anything with it. ;-)
I think if there's any slope to the barrel a pipe cutter wouldn't work properly, the ends of the cut wouldn't match up correctly.
What ever you choose to do have fun with it and good luck.