Totally agree with Devrij, 693 all the way. Reason being that if you use a 689 for example, and your paint is a snug fit when you test it out at the beginning of the day, by lunchtime when it is alot warmer out, the paint will be bigger, and of course not fit anymore. You will tend to then get barrel breaks (paint breaks in the barrel) and this can lose you games.
Plus no matter who makes the paint, you always get that one ball thats a slightly larger size or slightly mis-shaped, and you get a barrel break that way.
You will get told that by matching your barrel size to your paint will increase accuracy and efficiency, and it does, but its marginal. To the point where its pointless. You may add 30 shots to your tank of air for example, but if you break paint, and cant shoot the enemy, what is the point? All for 3 seconds more firing. Id rather shoot paint all day.
edit: and you beat me onasilverbike