Don't underestimate the stress produced by the motor and the moving parts in the mechbox (The old Tokyo Marui AEGs are infamous for that). The easiest way would be to use existing parts for the operating devices as much as possible. For example, have a look at AEP mechboxes and motors. You'll likely need a custom hop up, since you're inserting BBs from the side, but anything that you can just integrate might be a win.Do you really need to do those? I wouldn't have thought that would be necessary unless you're making the gearbox or something out of plastic.
Goes not just for the SMG, but also a BR or AR