While I agree with all the reasons given for matching your barrel's ID to your paint size, I would like to point out that If you have two paintballs, of equal weight, and one is smaller than the other, the smaller one will travel further (the wind drag varies with the cube of the velocity, that's a rather harsh drag curve) than the larger one if they both start out at the same velocity and travel angle (vector). That's why some tournament grade balls are smaller and with a thicker fill (when Diablo was first came out, both Inferno and hellfire tended to be supersmall...at least at the first tournament that we were sponsored by them). Some manufacturers were getting rediculous about fill weights and ball size and so some of them voluntarily limited fill weights so the balls wouldn't have enough mass to crack mask lenses or hurt people.
In my experience, most rec-ballers will rarely see supersmall paint. If anything, their paint will be on the large to medium size.
If you shoot an open bolt gun, you can get away with three barrel Inner Diameters and the larger and smaller ID's will probably rarely get used. I know a few Angel users who don't even know what their barrel ID is...they just deal with whatever happens to their paint 'cause they don't want to think about paint size. I think this is why openbolt guns got a bad rep. for accuracy in the beginning (starting with Automags), people didn't NEED to match bore size to paint size if they shot an openbolt gun...while 'Cocker users HAD to match bore and paint sizes 'cause their balls would roll out of their gun if they didn't...or they'd get plenty of breaks in their barrels.
As a side note: If you have an Intimidator and it's eye is in Classic Mode (it will shoot with a short delay if it senses there's no ball in the reciever) and you have a tendency to be jumping around or running as you're shooting, you have a chance of still chopping a ball with your bolt 'cause the delayed fire can catch a ball before it's fully seated in the reciever. I verified this, this past weekend. If you have a WAS board, set the eye in FORCED mode unless you're a backman. It's pretty rare, as the Wicked Air Sportz manual points out, but it does happen...I'm a believer
Ray "I snap hoppers" N.