Ok, you asked for it. The ball would slow down a few fps due to wind shear and factors such as lower air pressure. However, there will be no problem with air resistance. first of all the ball is small enough and going slow enough when it leaves the barrel for this not to matter. Also it is a ball. Therefore round. There has already been a post on another thread about how balls do not change shape (I think Kodak did a study). If there was enough air resistance it would go eliptical.
As it is fired upwards, it will lose its speed due to gravity, and reach its highest point. Then it will return, and accelerate at the same rate as it decelerated on the way up, hitting the ground at roughly the same speed that it left the barrel. It wont reach a terminal velocity (as a body would) because it has already gone at a speed that is going to be higher than that attained by it falling, just from being accelerated down the barrel.