Which muzzle flashes? It depends a bit where you are starting from. For example, there are a bunch of preset muzzle flashes labeled AX-series that are totally animated already to simulate automatic fire. You can look at the keyframes for any one of them to get an idea of what settings to adjust to get a similar appearance. Simply changing the Rate of Fire control will change the percentage of frames on which the effect is actually visible, but I find that to get realistic results is important to change up some other settings as well, to get greater variety in the size and shape of the flashes. So I prefer to manually keyframe the effect on and off and adjust the size of the flash, because I like the results better.
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