Thats a physics question. I can't tell you the answer but I know that the air resistance and gravity are what slows the bullet down. As for the speed on the way down after being shot straight up has to do with the mass of the bullet, the distance trveled (down) and the acceleration of gravity. The bullet would travel in a staight line forever in a vacuum like outer space, but gravity and air resistance are forces acting on it to slow it down.