I am building a 40 cal. slug gun and have started making breech plugs.  One is of 4140 annealed steel and the other is of 1018 CDS.  The barrel is a Green Mt.  40 cal. 1 in 16 and also made from 4140 steel.  I plan to configure the plugs after the Rigby style with a clean out hole on the right side.  I am aware that the 1018 CDS plug will have to be case hardened which may present a problem for me to do this correctly.  The question is can I made the plug from 4140 and forego   the heat treat.   Much the same as if I were building a under hammer gun and screwed the nipple directly into the barrel.  I am concered not so much for myself  as I will mould bullets less than 410 grains and keep the charge within reasonable limits as I am only shooting at ranges of 200 yds.  My kids and grand sons will some day shoot this gun and I want a reasonable margin of safety.