You should use metal bushings (or larger ball bearings) whenever you want to go beyond 350FPS or beyond (meaning anything stronger than a M100). If you use a hard and short spring, you should switch to metal bushings. Note that shimming must be done all over again when you install new bushings.
When installing new bushings, make sure you have them properly aligned with the place holders. Additionally, we recommend that you glue them in place, or otherwise they may go loose and spin together with the gears after prolonged use.
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