Moonclips are faster to use than a speedloader and allow quick and reliable ejection of the fired rounds, with a quick reload of all chambers at once.
Moon clips are a reloading device/method that makes it much easier to load a revolver cylinder with all your cartridges at one time. No fumbling, while loading your cylinder with rounds individually and possibly dropping some of them on the ground. No speedloader to pick-up off the ground after you load your revolver cylinder.
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