What James K says. Stainless is rust resistant, not rust proof. It must still be cleaned and oiled.
After cleaning the gun, you might want to appy RIG Gun Grease on the affected part. Rig will provide a barrier between the metal and the corrosive effect of sweat. It doesn't hurt to wipe down the gun occasionally with a rag treated with gun oil.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!