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With PORTED cylinders there is a hole or "port" in the cylinder allowing air to escape as the spring pushes the piston down the cylinder (for p90 i think the hole is 1/4 way down the cylinder) with no compression resistance the piston accelerates very quickly until it hits the unported section then there's no where for the air to escape but out the barrel. UNPORTED cylinders do not have a hole in the cylinder so the spring pushes the piston with compression resistance from the start. Basically it doesn't get a running start ramming itself into a cylinder of air. When your bb leaves the barrel the piston had only accelerated about 75% of the max speed. That`s why you get a slower fps with a shorter barrel and maxed out fps with the longer barrel. The bb is allowed to accelerate longer.
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