The Custom Products barrel back is one of the two pieces needed to use the Custom Products two piece barrel. The Custom Products barrel back comes in 5 different bore sizes and is available for all paintball guns on the market. When selecting a barrel bore size for the barrel back, be sure you choose one that will snugly fit your paintballs. The more expensive paintballs on the market tend to be smaller in diameter. The larger size paintballs are usually the less expensive paintball or have imperfections so you would want a larger bore barrel back. If you are not too sure as to the size barrel back you need, we recommend going with the .689. The 689 Custom Products Barrel Back should work great with pretty much all brands of paint as it is right in the middle of options. Overall, this will give you the best accuracy as well as consistency out of your paintball gun.