Originally Posted by jeepejeep
Doesn't need to be a vacuum. Gravity pulls the same no matter what just as others have said here.
Yes, you do need the vacuum. Air creates resistance that takes a greater effect on a lighter object. Don't believe me? Drop a piece of paper and a rock at the same time. The air resistance restricts the paper's fall. In a vacuum, there is no air so the resistance isn't there.
Also, a bullet actually arcs upwards as it leaves the muzzle. That bullet would have a further distance to fall, so would probably hit the ground last.