Wilbe , I don't no what factory ammo is avail. for the 375 jdj , but its so easy to reload for I won't mess with factory or custom stuff . just get your self some new 444 marlin cases - run them through your 375jdj full lenght rezing die and load them up , ITS BEEN A WHILE SINCE i STARTED OUT WITH MINE BUT i AM PRETTY SURE THERE IS NO FIREFORMING NEEDED. HORNADAY 220 FOR DEER SIZE AND HORNADAYS FOR ELK SIZE GAME, HAVE FUN maybe some body can confirm the fireforming question.