Make sure that after the game you dry off the gun and store it with good airflow; inside cases is a bad idea, even if dry.
If small shallow spots of rust appear wipe down with WD40 and then apply a bit of good quality gun oil, but don't drown the thing. Easiest is to apply a few drops to a paper towel and apply it like that.
If there are screws and you can remove them, store them in a bag of WD40, vinegar, or any rust removal product for a day or so, dry them off completely and apply a bit of gun oil.
For larger parts, or deeper rust get a rust removal/wire brush and whatever I mentioned earlier, and then treat with gun oil after.
Guardians of Asgaard - KF43 -
VFC KAC SR16 Geissele SBR
VFC H&K 416D