All this data is potentially out of date, and should be taken with a truckload of salt
Range bonus is a percentage value specifying the bonus (negative or positive) that the ammo will incur when loaded into your guns.
When using eg. small 150mm rails, which have a optimal range of 12km, using antimatter charges would bring this down to 6km (Range Bonus -50%). Using lead charges would keep it at 12km (No Range Bonus). Generally the more damage the ammo does, the shorter the range.
See also Items Database: Ammunition and Charges:[]