Your "buy in" is the purchase of ammo. Your health bar is your wager. You lose money for damage taken, and gain money for damage inflicted. This is the basic concept behind the first true "gambling" First Person Shooter, Kwari, and it seems like a mild adaptation of the basic concept in poker to a completely new game and genre. This is truly a game of skill that you wager upon.
However, while the concept is a step beyond the model employed by sites such as Tournament.com, the legality is still questionable in the United States. As I've pointed out before, the Unlawful Internet Gambling Enforcement Act ("UIGEA") was created, at least in part, with online poker particularly in mind. Hence, the Skill Game Protection Act ("SGPA") is proposed to specifically exempt games for skill, which includes poker, from the UIGEA. However, the SGPA hasn't passed, and so therefore it is reasonable to assume that the UIGEA still encompasses games of skill until the courts say otherwise, even though the UIGEA is quite ambiguous on the matter.. As such, even though Kwari is purely a skill game, it may be covered. But, the overwhelming ambiguities in the UIGEA may provide Kwari with a loophole, or the SGPA would almost certainly exempt Kwari from the UIGEA.
There's also a bigger tax implication for players. If you are a master of Kwari and can walk away with thousands of dollars a month, is it ordinary income or gambling winnings? I think the argument can certainly be made that winning in Kwari is no different than what, say, Tiger Woods wins in golf or what any other professional athlete is paid. On the other hand, poker winnings are gabling winnings in the eyes of the IRS. The classification of Kwari winnings could easily go either way, and mean a significant difference in taxable income depending on the result.