The most coomon upgrades increase velocity (FPS) and accuracy. On an AEG the best way to do this is to upgrade the spring to get more power, upgrade the battery to increase the torque of the motor (due to increased power need to stronger spring), and add a tight bore barrel. The tight bore barrel is precisioned machined to make it as tight as possible while still allowing bbs to pass through with as little friction as possible. This lets less air pass around the sides of the BB and increases the velocity.
External upgrade kits usually enhance the appearance of the gun or allow for more rail accessories to be added. Rail accessories may include red dot sights, scopes, lasers, or flashlights. Other parts might be tactical forgrips on rifles, bipods, or magazine adapters.
