posted on May, 17 2010 @ 07:33 AM
This is really a vector problem...
(Assuming a gun with a barrel of 0 length, and no wind or other resistance.)
If the gun is fired horizontally, there is an external force applied by the gun in a horizontal direction. However, there is no external force applied
by the gun in a downward direction. The only downward force is that of gravity.
Therefore, the bullet will move much further horizontally, since an external force is applied horizontally, but they will both move downwards at
exactly the same speed, since they have only the same external force (gravity) acting on them in the downwards direction.