Range weapons usage come in three formats: simple (x), extended (x/y), and complex (x, -z). As is always true, your GM can modify this or any rule for any item to suit the game.
Simple (x): means the maximum range of the item is the x amount. Anything over that amount is an automatic miss.
Extended (x/y): means the maximum range is normally x, however it can be extended to y by cutting the attack chance amount rolled in half. If this amount is a fraction, then round down to the nearest whole number.
Complex (x, -z): means the normal maximum range is x, but this can be extended by subtracting z from the attack chance for each time x is added to the distance.