NOTE: I've put down only rules for getting hit. You can customize the rules for ammo stealing, balls as grenades etc.

Team Tag Nerf War

   This game is best with 3 or more players on each team. If you are hit by the enemy team, then you are frozen. You can't move or shoot your gun when frozen. You are unfrozen when a teamate tags you. When all of a team's players are frozen, they lose.

Hit Point Nerf War

    This game can be played with teams or with individual players. It works best with players or small teams, 3 per team max. Each player has a certain number of hit points, say 10. Every time he is hit, one HP is taken off. (Varition: guns like Big Bad Bow can do 2 damage) If the honor system won't work, use index cards or something. When a player loses all of his hits, he dies. If playing with teams, a team loses when all of its players are dead.