Never run out of ammo
This is quite simple and does not require a lot of work. To never run out of ammo in a mission after you kill a Nazi pick up his MP40, MP44, or Kar98. They are the German weapons and all enemies carry one of the three. After you have 2 of these(I suggest the MP40 and the Kar98 because then you have a rifle and a powerful SMG.) you can just kill the Germans without having to worry about ammo because when you run over one of the dead enemies you pick up his ammo.
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