Buy a PC?
By far the biggest range of shoooting games, and you can play various custom maps and mods for free rather than the rip off DLC console packs.
You could try fallout new vegas its an FPS/RPG but will get you hours of play (offline)
Im not sure what to suggest because everyone goes for CoD and its one of the most average shooters ive played, but hey i must be wrong because so many people play it, its not like there were 1000 games before it that were identical. (4 player split screen with mates is good though)