Choose paint that matches the ID of the barrel ( ID is the Internal Diameter) and stick with that (the only way to check is to try and blow a few balls through the barrel, they should go through fairly easily.
Originally posted by pez069 what if u have to use the paint your site has and they do not allow u to bring your own.......does the size of the ball make that big of a differencE?
Getting back to a bit of reality here, yes it can make quite a bit of difference. It's not so bad if the bore is too big for the barrel - that just turns your marker into a bit of a gas hog & affects the accuracy. If the bore is too small however, you will get constant breaks in the barrel & possibly even blockages which can ruin your game or even whole day.