as a general practice, when we get our shory barrel done at dlask, the finished barrel is 18 3/4, a 1/4 inch over the silly non restricted barrel rule.
this is manily done in case of possible chrome flaking while turning in the crown and gives the machinist some working room to be sure the barrel finsihes over 18.5
also f you use the chop saw method to remove the barrel end, cut it right behind the flash hider so as to give your machinist lots of room to work and so the heat of the chop saw is not near the site where the barrel will be finished at.