What happens when you fire a bullet? You're converting the energy stored in gunpowder into gases that accelerate the bullet down the barrel... But that conversion of energy states also produces noise and light in addition to propellant gases...
Take these two tanks/artillery guns.
This is essentially the same distribution of muzzle gases as a bare barrel:
And this is basic brake distribution:
Basically, w/o a brake, you're sending gases, noise, and flash all downrange. With a brake, you're compressing and redirecting those same gases and sending them in a direction that's closer to you...
Hence, why some of CQB operator types still run brakes, but run linear brakes... Allen Gun Works sells a linear brake out of Canada for the VZ58:
http://www.allengunworks.com/products.htmlThe special forces brake and Zendl brakes both have braking and flash reduction capabilities and both have been approved for Czech military use.
In a gun fight the speed and accuracy of rounds down range matter, and brakes excel in this respect.
And flash hiders only work for the visible light spectrum... In night vision, even with a flash hider your flash will be visible. And even w/ suppressors, your first shot is more visible and noisy as the propellant gases burn off the oxygen in the can... Followup shots have less flash and noise... (Point being, for night ops, you really need a suppressor not a flash hider on the modern battlefield, but even that has its limitations.)
There are a lot of good youtube videos showing above, and I posted some less than a month ago to a similar thread here if you care to search.