or just buy paintballs that come in the size of your barrel. the bore size of the barrel should have been advertised when you bought it and paint says the bore size on the box, if you don't know what bore size your barrel is then get a shop to gove you a ball in each bore size (most shops will do this if you buy a case of paint afterwards) and blow them through your barrel, if it rolls through, too small, if it won't budge without physical help, to big
. if it blows through, then it fits just right.
hope it helps