Some weapons in any time period can be used over distance. These weapons need to be assigned a range. At close range, a weapon can hit fairly easily. At longer ranges, the shooter would have more trouble as he is limited by the ability of his eyeballs. This feature has changed little over the course of the history of warfare. With this limitation in mind, the maximum effective range of most modern weapons is about 300 yards, even with weapons used today. The use of scopes and other visual enhancements could certainly extend ranges for ultramodern and science fiction combat to the actual range of the weapon being used. When we consider smooth bore muskets and ancient weapons, this range is reduced to just 100 yards. Close range for most weapons can be simplified to about 50 yards.
At close range, you can assume that the shooting unit has the same chance to it its target as a unit actually in melee. Saving throws should be granted in the same manner as hand to hand combat. If a missile armed unit is charged and has not shot in that turn, it may take 1 close range shot before the melee ensues.
At long range, a penalty of 1 should be applied to the hit roll. This penalty, of course, is because it is much harder to hit a target, even a group of people, at longer ranges. This will effectively cut the chance to hit in half.