I assume you are referring to the A2 flash hider on the Sport model.
There's a bit of a story behind the A2 so I'll go through it quickly. Early models of the M16 used a three-prong open flash hider. This was found to snag on branches in field use, so a closed "birdcage" flash hider was developed for the M16A1. The A1 flash hider had ports all the way around and did not have to be indexed. It was a good flash hider, but would kick up dust on the ground when firing in the prone position. Later for the M16A2 and all current variants, the military went to the A2 flash hider. It does not have ports at the 6 o'clock position and must be indexed when screwed onto the barrel by means of a washer. The military advertises this as a combination flash hider and, yes, muzzle brake.
While it does provide a slight braking effect, it's primary purpose is hiding muzzle flash. The A2 is a very good muzzle device, even with all the fancy options in use today on the custom market.
A true brake has only one purpose, and that is directing gas to limit muzzle rise, at the expense of blast to the sides and increased noise.
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