Yep, Warhead's developers sat down and tried to think of ways to fuck up the game for anybody who illegally downloaded their copy and came up with "all guns become Chicken Gun." It doesn't matter which gun -- you're spraying chicken bullets like it's going out of style:
If only they'd included a physics engine that allowed the chickens to kill the enemy with their blunt force trauma.
No matter how many times you upgrade, the results are the same: Chicken Gun. You don't even get bigger chickens or anything like that. It is a thing of beauty:
In case you're wondering, chicken bullets do no damage whatsoever, meaning you can't kill anybody, and the game cannot be won. You can't die, either, because the bad guys use chickens, too. You could seriously "play" this game for all eternity and never get anywhere, you and your enemies endlessly bouncing streams of poultry off of one another in a sad metaphor for the futility of warfare.