The velocity of the moving plunger is what dictates FPS. A heavier stiff spring will accelrate the moving plunger faster then a stock heng long. However a shim inside the moving plunger has the same effect. It compresss the spring more, which in turn means more potential energy. Ultimately you are limited by how much torque can be delivered through the motor and nylon gun gears. With stock a spring and shimming I have been able to exceed that limit(.ie sheared teeth of the drive gears), so I doubt much more can be done with a spring. However if a guy could enlarge the cylinder diameter or length, you would expect greater velocities as more compression would occur.

Another thing the dictates the speed on the plunger is how fast air is allowed out through the nipple that sets the 6mm ball in the rubber sleeve. Opening up this hole allows the air to escape the cylinder faster. I did this on my latest gun, which is earmarked for my SU100, and the result actually dented a soup can from about 5 ft when fired on my work bench. As anybody who has worked with these gun will agree they are very finicky. How much the BBL compresses the rubber seal also has a direct relation to performance. Compressing the seal too much increases the friction fit against the pellet. To little and the seal is not tight enough and allows some air to escape uselessly within the gun. For this reason it is very hard to determine what modification has the greatest benefit. Any varible that gets changed slightly has an effect. I am going to try opening up the bore on the plunger nipple on other guns to see if this gain is measurable and repeatable.