Gear Shimming is a process, but more of a skill - a highly technical and expertly crafted art, one might say, that distinguishes a technician from a hobbyist. Mass-produced weapons often have poorly done shimming, so despite the higher initial cost and "upgrade parts straight from the box," they last only a few thousand rounds.  

It involves an activity where shims of various diameters are inserted between the gear and bearings in the gearbox to eliminate play between the gear and the gearbox. However, you must not over-shim the gears, as it could cause friction. So, using the trial-and-error method, you add and remove shims often by 0.1 mm until you achieve the ideal shimming. In this state, the gear has minimal to almost no play in the closed gearbox (to be tested in a fully closed and tightened gearbox), and it spins freely with minimal resistance (gears should not be lubricated with vaseline or other oils during testing).

While shimming individual gears, you must also ensure that gears do not rub against the gearbox skeleton or each other. Therefore, you need to check the height of individual gears relative to each other. Moreover, you must also watch the position of the motor gear in relation to the motor pinion to prevent the motor gear from being too low, causing it to misalign, or too high, risking contact with the gear edges. And we're not done yet; you also have to pay attention to the position of the piston gear relative to the piston teeth to ensure that the gears pull the piston in its center and not at the edges, preventing contact with the casing or damage to the gear tube.

Some technicians start by shimming the sector gear since it is lower than the other gears, but this method increases the risk of the motor gear being improperly positioned relative to the motor pinion. While enough teeth usually touch each other, preventing immediate damage, it results in a worse motor sound, higher battery consumption, and reduced gun rate of fire. The alternative method is shimming from the motor gear, requiring the motor gear to be high enough not to rub against the sector gear and simultaneously avoiding friction with the gearbox. Both methods have their proponents and opponents, along with advantages and disadvantages.

In conclusion, gear shimming is an art that distinguishes amateurs from professionals in the AEG upgrading domain. 

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