I wanted to know because I am considering buying one of them for my M4.
gtamayo is pretty correct except the part about the flash supressor. Most guns like AR15's have flash supressors already on them and what they do is redirect burning powder so it does not all come out the end of your barrel and give away your position when firing. Many of them only have ports going to both sides and up which does 2 things. First it keeps your barrel down when firing and second i keeps dirt or sand from being thrown up when firing prone. Flash supressors are not the same as sound supressors and are not NFA items. Most AR15's or M4's already have them on the end of the barrel unless the gun is post ban configuration.
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