Corrosive ammo leaves a residue of Potassium Chloride on parts of the gun. In the presence of air, which contains oxygen and moisture (humidity), the potassium chloride will speed up the corrosion process.
Shooting non-corrosive ammo will physically blast away some of the potassium chloride, but it's highly doubtful you'll get rid of it all, and therefore the gun will still corrode.
For the inside of the barrel, this might actually work, because the friction of the bullet traveling down the bore could remove most of the potassium chloride. However, it probably won't remove much of it from the chamber, receiver, gas tube, piston, bolt, etc... Seeing as the barrel is one of the easiest parts to clean, and you'd still have to clean all that other stuff anyway, you're really not saving much work by firing some non-corrosive rounds.
On a semi-auto, like an SKS, you'd definitely need to clean all the fiddly parts that have been contaminated by escaping gasses.
On a bolt-action, you might be able to get away with only cleaning the chamber and bore. But that's so easy to do, why even look for a way not to?
