I shoot airsoft at the local fields all the time. I would recommend at least a clear filter over the end of your lens, and if possible, create a custom lexan shield for it.
Optionally, go for a large zoom and stand way back! Keep in mind this is a game others are playing and trying to win so you need to be as unobtrusive as you can, as giving away a position for a shot will piss people off royally.
Walk the field for a bit, get to know it and the spots where the action will take place. There are always natural chokepoints this occurs on. Also, for technique, practice panning if this is not a skill already. Be visible, don't be afraid to vocalize your presence and above all post you shots, I'd love to see them!