Light is affected by gravity but you're right, there wouldn't a noticeable effect. The idea is that bullet drop would affect whatever weapons should be affected and not any hyperspatial or time-bending guns that are unaffected by gravity. Technically all plasma weapons in Halo should be affected, each live round is enveloped in a magnetic bubble that can be guided (usually this technology is reserved for space battles) to an extent and pops like a water balloon upon forceful contact. Plasma is still affected by gravity because it is a state of matter. Most any projectile [i]should[/i] be affected by bullet drop unless their is some canonical reason it is not: space-magic, high-tech, or an auto-adjusting scope.