We chat about videogames and videogame accessories.

First thing I think of upon hearing "guns" and "decks" is the ridiculous inventories to be had. You just draw a gun from the deck, unload it into whoever, and when your ammo runs out toss it and draw anew. It seems to be what happens in action movies much of the time in any case.

In my experience customizable guns don't get much sillier than in Resonance of Fate. I didn't think much of the story, but the gameplay is, I think, unintentionally hilarious. Like, it wants to be this super-gritty, hardcore American action flick, but it quickly devolves into Ridiculous Backflips: The Game, co-starring How Many Barrels Can I Tetris Onto This SMG. There are also some neat world map mechanics where you have to unlock individual squares and connecting certain tiles with the same color grant bonuses, and there's also outfit customization, but that's not what you end up remembering.

It's not an FPS so it's not quite what you're looking for, and I don't know if I'd recommend it exactly, but it's a spectacle in its own ways and looking into it might help flesh some of your ideas out, if you actually want to pursue this.

E: This vid is probably better for a full suite of Things That Can Happen, but the player's... not great.