For jobs that you want to get the part into the gun and back in service quickly 4140HT is a good steel, HT means that it is already mildly hardened. Not so much that it is not machinable but enough to make parts with the necessary hardness and wear resistance. Oil hardening drill rod is another steel which can be quickly hardened and tempered with an acetylene torch, parts made with it and drawn back to a straw color for toughness and a blue where more resliance are needed make very serviceable parts.
Probably use those two steels along with A-2 and S-7 for most parts making in the gun shop.
