This been in my head for a while now, there would probably be some sort of board involved for once. This game could be potentially be played as a solitaire because it was initially thought to be a Zombie Survival game.
You the Player choose your Gun of choice and a deck of ammo. Each Gun has an ammo limit (ie a Hand maximum). Each turn you load your gun up to your Limit (e.g 6 cards for a revolver). Your can only load your gun with the corresponding ammo (so no stuffing Rockets into Pistols). Your turn ends after you've used all the cards in your hand (thematically your Opponent's turn is being taken during the time you re-load your gun].
Each bit of Ammo should deal damage (to whatever the hell your fighting, be it a player or a zombie) potentially along with side-effects.
In addition each bit of Ammo has a Critical Hit Ratio written in X/Y where X is the number that needs to be rolled and Y is the size of the Die rolled. Potentially, some extra damage is inflicted here or abilities of ammo only activate on a critical hit.
Gun's also could, probably should have a range and a weight. Range is how far the gun can shoot whereas Weight is subtracted from your Character's move points (So you can't sprint easily with a Rocket Launcher on your back)
That's the gist of the idea anyway, a few standards in there like Health and Move, you could play against an Opponent in a PvP manner or play against a hoard of zombies all by yourself (in addition to this version, the bigger and louder your weapon, the more zombies that will appear).
Could probably also make it so you can find different weapons on the way (though only being allowed to carry one main with an extra add-on (knife, tazer)