Ben -you know more than most people but here it is for the record:
The male (and female) threaded part of 1/2-14 NPT has to have the same geometry -across the board- for the typical takeup of .4375 (7/16"). SCH 40 would have an ID of about .622" (~5/8"), an O.D. of .840" So your first thread could be roughly 11/16" (.688) -Now that is a 3/16" Dia. stretch -it can't be fun putting those on! ...and it is going to leak because there is nothing to fill in the void between threads.
Material shouldn't make a difference for american standard pipe.