If it were going to do that much damage and move that fast, it sure had better be a large weapon, and take a *whole* lot of energy to fire and have a *very* slow refire rate.

How about it moves at 500m/s, doesn't do real damage the first 500 m, but some sort of trail is visible nonetheless. It's like firing some small object that amasses energy as it travels. You would see it coming from the moment it's fired, but it would just be a harmless low damage streak of light that increases in damage dealing potential as it travels further and furter.

The logic behind it might be something like you fire something out of another quantum existance or whatever, that gradualy shifts its existance from the other quantum level to ours during it's travel time. At close range it's still mostly in the other quantum level and has no real impact on our quantum level, but after 500m it's fully existant in our quantum level and packs quite a punch.