There's probably enough loose change in your sofa right now to get in on the latest Bundle Stars deal. $3.98 will get you eight first-person shooters, with a nice variety of games that are basically nothing but shooting, and some with a little less shooting, such as Zeno Clash 2.
You have a whole 28 days to pull the trigger on this one. I can personally vouch for the quality of Hard Reset and the Painkiller games, both of which are very straight forward, throwback, "shut up and kill everything that moves" kind of games. Do note, however, that Painkiller Overdose and Black Edition are from 2004 and 2007, and not the newer, better looking Painkiller Hell and Damnation released in 2012.
Zeno Clash and Zeno Clash 2 are known for their melee combat and bizzare art style, and Air Buccaneers and Nuclear Dawn are both multiplayer games with interesting twists. Air Buccaneers has you and your team manning a slow, lumbering battle-balloon, while Nuclear Dawn, which is also the best reviewed game in the bundle on PC Gamer, combines FPS and RTS elements. We thought it was "surprisingly deep and well-designed."
I haven't played Chaser, released in 2003, but for $0.49, there isn't much to lose on the gamble.