This step is not necessary for those trying to access the gearbox.

To remove the hop-up and inner barrel remove the screws shown.

With hop-up and inner barrel out you can separate the hop-up from the barrel. First slide the hop-up to it's lowest setting. Now removing the plastic clip piece.

Slide off the hop-up to expose a green rubber seal piece called the bucking.

Warning: Do not loose the hop-up nub. Which is a very small cylinder shaped piece that fits onto the hop-up arm.