Sorry I mean 'shimming'!
Shimming refers to the practice of adding or removing shims (thin washers) to the gear axles in order to improve gear meshing. This is important to reduce friction while ensuring maximum gear tooth contact. Correctly shimmed gears run quieter and provide less mechanical resistance, making your AEG more efficient.
AOE stands for Angle of Engagement, and refers to the angle at which the sector gear and piston engage. In stock AEGs they engage at the wrong angle, putting too much stress on the first sector gear tooth and the first piston tooth. The angle of engagement is altered by adding a spacer to the cylinder head, which stops the piston slightly further back in the gearbox. This means that when the sector gear engages the piston, the teeth meet at a different angle, one which puts less stress on the teeth. A quick google will turn up some pictures which explain this clearly.
These are just two of the methods you should be using to correctly tune an AEG for reliability. If you want to just increase the FPS, on the understanding that it may not last too long, you can just drop in a stronger spring and see what happens. But a stronger springs puts more stress on the internal parts, so if you want your AEG to be reliable you should really do some research on good upgrade techniques. I'm writing a beginner's guide to AEG upgrades, but the information is all on google.
