When picking a paintball barrel, the most important thing to look at is first barrel thread to make sure you have the correct barrel for your gun, then comes bore size. Bore size is jargon for the inside diameter of the barrel.
The first thing that most people think is that paintballs are all the same size (.689 caliber), and while generally, this is true, paintballs can change in size. Paintballs can vary in size from manufacturer to manufacture, quality and climate can all affect the size of a paintball. While the size change is a very slight change for example right now in mid 2020 .685 is the most common paintball size.
Getting the correct bore size that fits your paintballs will greatly improve accuracy. While getting a bore size that fits your paintballs is important, it is still more important to use quality paintballs for best accuracy.
The best way to find the correct bore size is to size match your paintballs to the barrel you are using. This is done by placing the paintball in the barrel if it rolls out the barrel is too big if it gets stuck and you have to push it out it is too small. Ideally, you want the ball to get stuck but be able to lightly blow it out with your breath.
If you are going to be getting one barrel we would recommend something that is .684 or .685, or better yet something like a GoG Freak Barrel Kit would be even better. The full Freak Kit comes with 8 different bore sizes to help get the perfect fit every time.