Two bears,
They reduced recoil, the disadvantage is increased muzzle blast.
These are "installed" by porting (drilling or cutting angled holes or slots into) the barrel itself. Such brakes avoid the added length and the slight bulge at the end of the barrel of a screw-on muzzle brake. They are a neater installation and cannot loosen with use. On the other hand, they slightly reduce bullet velocity, tend to collect fouling, make the barrel more difficult to clean, and, most importantly, cannot be removed.
http://www.chuckhawks.com/muzzle_brakes.htmThanks Busta. I wasn't thinking about the corrosive problem with the cut or drilled holes but I did think about the cleaning problems associated with them. Much more trouble than they are worth.