A good mesh between the pinion and bevel provides a quiet gun with minimal squeal (Provided the tooth profiles match). I started doing this awhile ago as part of my shimming routine in order to get the pinion/bevel mesh as good as I could realistically get. The steps go as follows: 1. Cut a little window in the top shell so you can see the edge of the bevel gear. 2. Shim the gun, I generally start with the spur and get it as low as possible to provide as much room for adjustment as possible. 3. Close up the gun and test fire, adjusting the motor height until tightening just starts to add some high pitched noise back in, then back off 1/4-1/2 turn where the gearbox is the most quiet. 4. Check the mesh, if the edges mesh, it's about as good as it'll get, though if the gearbox is still noisy you can try moving the bevel up and down and see if it helps; bear in mind some gears just don't mesh with each other well and you'll need to play musical gears. In the above photo, the bevel is a little on the low side and may need a 0.1mm raise. However, if the gun is quiet, the mesh is likely fine. 5. If the pinion is not flush, either raise the bevel so the pinion can set further in or lower the bevel so the pinion sits further out. On V.2 guns, I have an extra M4 grip I cut and use to provide a window. In the above photo, the bevel is a little too low and could go up a 0.1mm shim. That's all there really is to it. Hope this helps!