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A bullet fired straight up will reach zero velocity and start a return due to gravitational acceleration. The falling projectile will reach a terminal velocity, which will be a combination of gravitational acceleration and air resistance. In a vacuum, theorhetically, the falling object would eventually approach its initial velocity, but air resistance will prevent that. Mythbusters made an interesting, ala Hatcher, test, and fundamentally agreed with Hatcher, projectiles returning to the ground can rise to 500 ft/sec, but not much more, and that for .50, with .30 coming in closer to 300 ft/sec. 200 fps is enough to cause severe injury.