There seems to be already quite a number of mobile games based on this theme, but I guess there is always room for more. This is All Boom, a new shooter for Android devices. Rather than FPS, this game players more similarly to the classic online title, Gunbound.
One feature which caught my eye was the location-based service, which can detect friends nearby to play the game in multiplayer mode. A casual and easy game to begin with, All Boom is suited for everyone with those few minutes to spare. An iOS version is apparently in the works.