I do some paintball play and I can tell you from personal experience that some barrels are more accurate than others regardless of length.

There's also more going on inside the barrel than simply guiding the ball through friction. There's an aerodynamic interaction between the ball, the propellant and the outside air that has to be balanced or accuracy will go in the toilet.

There are barrel kits available that have a replaceable insert for different size balls. Even they are all supposed to be .68 caliber, some are a little bigger, some are smaller.

One of the biggest influences you can have on paintball accuracy is using quality paintballs. I'm always tempted to but the cheapest paint I can get my hands on, but every time I buck-up and pay the extra $12 or so per 2000 balls, I'm always really happy with the result.