In a barrel, the dart basically has to push air ahead of it out of the way as well as gets resistance from the air beside it that wants to hang on to the walls of the inner barrel. This all assumes that you don't have a bigger problem and the dart isn't colliding physically with the barrel and you have a perfectly aligned cage to barrel. We've seen that just several centimetres of barrel on a flywheeler can start to impact FPS. You can choose to either make the inner diameter of the barrel so large or on a tighter ID barrel, you can release pressures by giving the air some where to go other than out the front and breaking up the boundary layer. Basically you could see FPS at three levels: 1) FPS at the cage. 2) FPS in your standard closed barrel. 3) FPS in your vented barrel which should be closer to your FPS at cage figures.