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Most fields and tournaments require you to use their paint. Standard paintballs are .68 bore, but the actual bore varies anywhere from .674 to .698. If your barrel is too small for the balls you're using, you'll break more paint. If your barrel is too large for the balls, you'll see a dip in consistency and in accuracy.
To get the best possible shot, a lot of paintball players use a barrel kit. Barrel kits contain inserts or backs that are designed to fit the exact bore of the balls you'll be playing with that fit into the barrel.