What is needed to make an AEG shoot ~400fps?
For 90% of the AEGs out there you will need a minimum of 2 items. 1: A Systema M120 spring (or equivalent spring). 2: Metal bushings. You can reference the mechbox guides on Mechbox.com to see how to do it, or you could contact Airsoftworx.com, and they will do it for you (for $40 which includes parts and labor).