I just finished cutting back the barrel on my SKS to 18.5"
I removed the front sight.
Cut back the barrel
Re-crowned with this handy tool from Brownelles;
http://ww w.brownells.com/gunsmith-tools-supplies/barrel-blanks-tools/barrel-chamfering-cutters/79-muzzle-crowning-cutter-prod628.aspx
Pick the right Pilot;
http://ww w.brownells.com/gunsmith-tools-supplies/barrel-blanks-tools/chamfering-tool-pilots/brass-pilots-prod18295.aspx
Make sure you get the 5/8" tool so that if you cut the barrel a little off square you can hand turn the cutter to square it off.
Then I cut the bayo-lug off the front sight. Re-installed the front sight onto the barrel, then I soldered it in place (this ensures that it is square with the rear sight and stays that way). Then I drilled through the hole in the front sight to notch the barrel and re-installed the front sight pin.
It looks a little crappy now. but I want to test it out at the range (function and accuracy) before I re-blue the barrel.
I'm a little concerened that the 2" shorter barrel will affect the pressure curve in the gas system....... If it does I'll have to tear the gas system apart and drill a larger hole in the barrel to get more gas into the tube.....
I was quoted $250 by a g-smith to cut back the barrel and re-jigger the front sight, so $125 in tools seemed like a no brainer.........