The cylinder is where the air gets compressed and forced out by the piston head, propelling the gel. The cylinder also dictates the air volume capacity and is a important factor to consider about when you build a gel blaster. Excessive air volume on a short inner barrel length setup can cause low FPS because there simply is not enough back pressure generated enough to shoot out the gel at full potential before the gel leaves the inner barrel. This is what usually referred to be the air cylinder to inner barrel air ratio.
100% Non ported : 35-40CM+
80% Ported : 27~35CM
70% Ported : 25~30CM
50% Ported : 13~20CM
Please note that these are approximate figures.