(taken from the ASGI website)
Upgrades
What upgrades can I do to a spring gun?
Most spring guns don’t have any upgrades available mostly due to being too low end.
What upgrades can I do to a gas gun?
Usually you can get metal slides, upgrade the valve on your magazine (hi-flow valve), upgrade the recoil spring to get a stronger kick out of your gun, and extended magazines (usually for the Glock series).
What upgrades can I do to an AEG?
There are numerous upgrades available for the various AEG’s on the market. However, there are some low-end airsoft guns that you are unable to upgrade such as UTG, CYMA, Well, etc. Depending on how much you want to spend, you can upgrade the gears, the spring, the piston, piston head, and so on to increase the rate of fire, the reliability, and the velocity of your BB’s for your airsoft AEG. Some guns have limitations on what you can upgrade due to battery capacity, barrel length, and gearbox size. We do offer upgrade services and packages on our Upgrade Service link on our web page.