These are key to gearboxes that DO NOT have the typical m5 screw holding a buffer tube/stock/spring guide to the receiver of many different aeg platforms. A washer and m5 screw would work, but clearance could be an issue. This works PERFECT for g36 variants, even for gearboxes other than Retro Arms. I used mine in a JG SL8 using the stock gearbox. Spring guide retention is important to avoid your gearbox locking up/wear and tear on the gears. If your GB does not have a hole for a spring guide/mounting screw, make one. Dremel/file a hole where it needs to be. Use half the shell, put in your spring guide and line it up straight to see where you need to file on each side of the gearbox shell. You can tighten it in place by using your fingers, followed by using a pair of thin needle nose pliers.