In the f/a guns especially there can be fairly substantial flash. It's very hard to judge flash in daylight but it becomes quite evident after the lights go out. So for that reason and the ever present CDI factor you will continue to see flash hiders applied to the MP5 platform.
They protect the 3-lug tip from getting banged up. If the 3-lug gets banged up you will have mounting issues when attacking a suppressor. I guess they add a little bit of weight to the muzzle and that helps with muzzle climb somewhat. They really do mitigate muzzle flash when shooting a burst in low light conditions. Plus they look cool.
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