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The M4/M16/AR15 is the most popular airsoft rifle in the United States. That's not surprising considering that it's the primary assault rifle used by the U.S. military. So chances are you probably have an M4 rifle or its variant. This article shows you how to disassemble this rifle. From this point forward, we will use the term M4 to identify the M4 rifle and all of the variants.
The receiver is the main body block at the center of the M4 rifle. The receiver can be separated into two: the upper and lower receiver. The upper receiver is the attachment point for the front rail/handguard, the external barrel, the inner barrel, and the hop-up. The lower receiver carries the gearbox, the rear shoulder stock, motor, and handgrip.
To separate the receiver, tap out the receiver pin at the front of the receiver, right under and behind the rail/handguard. The photo below shows the receiver pin tapped out slightly.
Once the receiver pin has been completely tapped out, you can pull the upper receiver forward, while pulling the lower receiver backward. On some M4 airsoft guns, it is very tight. You can put the forward assist button to good use in this case.
The forward assist button is the round button on the right side of the upper receiver. It does nothing on an airsoft gun (at least on most airsoft guns; some realistic training airsoft rifle make use of it). Hold the handgrip with your right hand. Hold the rail/handguard with your left hand. Use your right thumb on the forward assist button. The receiver should separate with some pressure.
On a version 2 gearbox AEG, such as the M4, the handgrip is attached to the bottom of the gearbox and houses the motor. The base of the handgrip has a plate with two small screws (see photo below). The big flat screw in the middle is to adjust the motor tension to the gearbox. Unscrew the two small screws to remove the plate.
After removing the handgrip plate, you'll see the motor and attached wires (see photo below). Detach the wires and remove the motor.
Next step is to remove the handgrip. There are two screws inside the motor slot (see photo below). Unscrew the two screws. Be careful that the screws don't fall into the gearbox. Once you've unscrewed the handgrip, you can remove it easily.
After removing the handgrip, you'll see the bottom of the version 2 gearbox.
To separate the gearbox from the lower receiver, you'll have to tap out two receiver pins with a punch and a rubber mallet. The smaller receiver pin is just forward of the selector switch. The bigger receiver pin is behind, toward the top from the selector switch. The photo below shows the two receiver pins have already been tapped out.
Next, you'll have to disassembly the magazine release. Flip the receiver to the other side. Use a precision screwdriver to unscrew the magazine release button. Be careful you don't lose the spring under it.
Next, you have to remove the bolt catch latch mechanism. Pull the gearbox out of the receiver slightly until you have revealed the latch (see photo below). Unscrew the screw that is keeping the latch mechanism in place.
Switch the firing mode selector to semi-auto; otherwise, it would be difficult to get the gearbox out of the lower receiver. Once you've done all that, you can now wiggle the gearbox out of the lower receiver. Be careful the wiring does not catch on the lower receiver.
Before opening the gearbox, see if your gearbox has a quick change spring feature. Photo below shows an APS gearbox with a quick change spring guide (changeable with a 7mm hex wrench). If so, remove the spring guide and the spring first. It will make your gearbox disassembly much easier.
Unscrew all of the screws on the gearbox (see photo below). If you can't remove the spring first, hold the gearbox shell down as you unscrew the last screw.
If the main spring isn't removed, use a screwdriver to keep the spring guide in place as you separate the gearbox. But if you have a quick change spring system, you can just lift the gearbox shell. The following photo shows the gearbox internals.