I know of at least one gunsmith on this Forum that drills a single hole in the top of the Muscle Brakes that don't have a hole on the top. Some downward force to resist muzzle rise, but not enough force to bend the barrel down into banana form. So that's what I did with my Mini-Mag Muscle Brake, and will also do with my 4-port Magnum Muscle Brake.
Darrel Holland's muzzle brakes have been drilled on top for a long time now.
So I think it's more a matter of preference than negative affects on rifle accuracy. I've also heard from gunsmiths that report that even if the hole in a muzzle brake isn't exactly concentric around the bullet's diameter, they haven't seen negative affects on rifle accuracy. That would be a greater cause of differing gas pressure action on the base of the bullet leaving the muzzle than a port hole drilled into the top of a brake - to my way of thinking.
Darrel Holland's muzzle brakes have been drilled on top for a long time now.
So I think it's more a matter of preference than negative affects on rifle accuracy. I've also heard from gunsmiths that report that even if the hole in a muzzle brake isn't exactly concentric around the bullet's diameter, they haven't seen negative affects on rifle accuracy. That would be a greater cause of differing gas pressure action on the base of the bullet leaving the muzzle than a port hole drilled into the top of a brake - to my way of thinking.