The muzzle brake is a device attached (usually permanently but not always) to the end of a barrel that re-directs expelled gases from the barrel rearwards to counter act recoil. It is found on most large caliber weapons, including tanks and artillery. On the Barrett M82 .50 cal SWS (Sniper Weapon System) it is rather large. The concussion of the redirected gases is enough to knock the fillings right out of your teeth, as I found out the hard way the first time I ever fired one. I like the M-24 better, it is a converted Remington 700 in .308 (Nato 7.62 x 51) but is a long action version instead of a short one like the normal .308
I've got one on my M700 30-06 Remington BDL. I don't get that "scope cut" on my head, anymore. However, I also changed scopes.
I had a Redfield Widefield (rectangular eyepiece) that kept cutting me. I went to a Leupold 3x9x40 and haven't had any problems since.