Currency is used to buy weapons. Guns are usually bought with Gold refined from xplodium. Most of the better guns, such as the Cataclysm X4, Space Disruptor, and Kraken are bought with rare War Bucks. War Bucks can be obtained 5 a day by watching advertisements from sponsor apps, such as Clash of Clans and The Hobbit. One can also complete offers powered by Tapjoy for warbucks, amount depending on the offer. Both gold and warbucks can be bought from the Bank in the Store by paying real-life money, hence the game's freemium business model.
In the sequel, Gun Bros 2, xplodium does not need refining and guns can be paid with xplodium.