Actually, bullets shoot "flatter" trajectories when fired upwards also. If you sight a rifle in at 200 yds, and fire at an angle downwards, OR upwards, the bullet hits higher than expected. Basically, gravity only effects the trajectory across the distance traveled horizontally, so a 200 yd shot at a severe angle, whether up or down, will hit higher than the point-of-aim, because the bullet traveled less than 200 yds horizontally. I agree that a bullet fired upwards will lose velocity faster than one fired downwards, and that the upward-fired bullet will EVENTUALLY travel a highly arched path. However, this won't really happen for quite awhile, likely further than would matter in AH. I've never checked into long-range trajectories, but would guess it would take upwards of a thousand yards to see a noticable difference. Remember that even a bullet from a deer rifle like a .270 or 30-06 can travel 6-7 miles distance if fired upwards at the correct angle. It's not likely it would lose so much velocity to see an appreciable arch in AH ranges, unless fired + or - 30 degrees from horizontal.
The idea that bullets will hit low if fired upwards, and high if fired downwards, is a common mis-conception. This is easily proven by shooting at severe angles up or downhill, or even by shooting an arrow downwards from a treestand. If you live in town, go throw a baseball upwards and you will see the same thing. Anyone who has hunted in the mountains, or from a treestand can tell you how important it is to aim LOW for angled shots, regardless if up or down
