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If you enjoy shooting massive quantities of nameless and faceless virtual soldiers with a dozen various real life guns (and who doesn’t, really?) then you should probably check out Overkill. It’s a great app for getting a quick action fix while performing boring and mundane tasks. Be careful though, as your play time can easily be extended much longer than planned with this particular app.
The best way I can describe Overkill is as a rail-shooter (Such as Time Crisis) in which you don’t change areas and can’t move or take cover. Enemies will pop up all over the area and have a delay before they can shoot you, and you have to race to kill them before they can do so. Each of the four areas takes you through several waves, with each wave leading to tougher enemies with shorter delays between violence, all culminating in a so-difficult-it's-just-silly Hell Wave at the end.
Upon starting the game, it can be infuriating as the first available guns have awful accuracy. Luckily they let you sample the stronger guns as you progress through the early levels. You can upgrade and customize each gun, though there aren’t many options for you. You can even take a picture of your gun and upload it to social networks like Facebook, which seems kind of odd given how limited your choices really are.
Every great free game has its own in-game currency system that can be purchased for real life money, and this one is no exception. Overkill Medals (OM) can be used to buy several guns/gun upgrades among other cool things. Some of the best guns in the game can only be bought with OM though, which you won’t receive nearly enough of just ranking up through the game normally. They clearly want you to buy some OM, but luckily it is pretty cheap, and there are TONS of ways to get OM for free, including downloading other free apps or sharing Overkill with your friends.