To tweak the maximum possible accuracy from your paintgun you have to match the barrel and the paintball to one another.  Different brands of paint and different brands of barrels have different dimensions.  With barrels, these dimensions are pretty much fixed by the manufacturer.  Paintballs are manufactured at an approximate average dimension but are subject to change due to atmospheric and storage conditions.  Due to the fact that weather conditions can change a paintballs diameter and sensitivity to compression.  It is not at all unlikely that a player striving for the best accuracy and performance would have to change barrel sizes during the course of the day.

    On this page is a breif list of known bore sizes of paintgun barrels and approximate sizes of certain brands of paint.  I will add to this list as the information becomes available.