It doesn't look as though anyone has mentioned climatic conditions yet, so I will
Ball size can change based on current weather and humidity conditions. It's a mostly accepted fact that the primary factor in accuracy is paint-to-bore matching. If it's a proper fit, then you'll see some good accuracy, while if it's bouncing around or being compressed in the barrel, you will see a marked decrease in accuracy.
One thing you can do is get a general all-around barrel, that performs fairly well with all sizes and conditions. The most common (that I know of) would be the Bigshot, Ceramic, or the Palmer's brass barrel. If you have the cash, a barrel kit would be ideal. Most barrel kits are good, but the JnJ Edge kit is the best performance for your money. Hammerhead barrels are also very good, but expensive.
Personally I own the Tippmann Straightline (Hammerhead) barrel, as well as a Bigshot and an Apex (hardly use it anymore), so i'm pretty much set. If I do buy another barrel in the future, it will be a Palmer's brass because I have read that it performs very well with all paints, and would thus make a good backup if my Straightline is having problems.