This guide will go over the reason for adding space in between your hop up and the front of your gearbox shell as well as how to do it.
Why is it necessary to do this?
The reason for doing this is when the air nozzle does not retract back far enough back to allow a BB to load into the hop chamber. It should be said that this is typically only pertinent when you add aftermarket parts to your gun. Although manufacturers offer aftermarket parts for a certain gun or series of gun, but that doesn’t mean those parts are going to be 100% compatible with your AEG.
When do I need to do this?
This is necessary if your gun is either not firing BBs or fires BB(s) every so often. When it does fire, it should fire fine. Just to be safe, manually drop a BB into the hop up chamber and make sure it fires every time.
Now you are ready to add material between the back of the hop up and the gearbox shell, material should only be added to the area of the hop up that makes contact with the gearbox. *Before doing this you can try and manually push the hop up forward so it is further away from the shell and see if it feeds, this can be done very easily if you have an M4 with an upper receiver that slides on and off. If it fires fine that just reconfirms the issue.
Preferably with a hard rubber e.g. neoprene, glue it to the back of the hop up. You really should not need much material (thickness) you will most likely have to cut down your choice of rubber to achieve optimal performance.
The goal is to add the least amount of material possible and still have the gun feeding. The reason for this is because if you add to much material there will be too much of a gap between the bucking lips and the tip of the air nozzle causing an air leak which will dramatically lower your FPS.