There's steep gap on the continuum of susceptibility to corrosion transitioning from corrosive ammunition to non-corrosive. Corrosive ammunition - black powder, potassium chloride primers, cordite propellants - are death to a gun if not cleaned off immediately after use. The best cleaning I am aware of is hot, soapy water, hot water rinse, and oiling. Once you jump that gap to non-corrosive you have much, much more time, since the only issue is the products of combustion holding moisture and potentially other corrosive elements in contact with the metal. Good practice is to clean and oil. Make sure you dry the bore before firing. That said, a un-cleaned gun last fired with non-corrosive ammo can go years and years without cleaning without corrosion, given favorable storage conditions.
Not all mil-supr ammo is corrosive, you should check before buying. For example, I am aware of no corrosive 5.56x45. I'm guessing you'd have to go back to the 1950's to find corrosive ammo from U.S suppliers, probably more recently for British, and even more recent for Southern Asian or Middle Eastern stuff; but I'm not sure about that.
