The ICS M4A1 Metal Body Electric Airsoft Gun is a high quality Airsoft gun that offers the user flexibility to upgrade easily and quickly. With its unique split gearbox design, the ICS allows you to remove the rear receiver lock pin and quickly slide out the upper portion of the gearbox that contains the piston and cylinder of the gun. The advantages of this are notable. The user can obtain a second upper gearbox that is build for a lower FPS. One upper can be built for outdoor play where 400FPS is desired while the other upper can be built for indoor use where 350FPS is required. This exchange can be done in less than a minute. Another advantage is the ability to exchange uppers in the event that you experience an issue, such as a piston failure, on the field. Within seconds you can exchange gearbox uppers and be back in action! Another unique feature on the ICS is a anti-reversal latch release built into the forward assist located on the right side of the receiver. This allows you to decompress the main spring in the gearbox when you are done using it for the day. This is also an advantage for anyone running a DMR (Designated Marksman Rifle). A typical DMR setup might be running 450FPS and have full auto disabled. If the user experiences gearbox lockup that semi only guns are more prone to, simply push the release button and the gearbox will be reset to the neutral position. The large majority of the gearbox internals are standard version 2 compatible so upgrade parts availability is great. Overall the ICS M4A1 is a solid, well engineered AEG with features not usually found on most Airsoft guns.