Muzzle brakes are not allowed in ANY NRA competition that I am aware of and that includes Service Rifle competition.
Here is the rule that governs all this:
3.16.1 Compensators and Muzzle Brakes - The use of compensators or muzzle brakes is prohibited. An extension tube that has been installed on the muzzle of a rifle to extend the sight radius shall not be considered a ''muzzle brake''. The extension tube must have an interior diameter of .5 inches or greater and may have 1/4'' x 1 " slots cut at 12 and 6 o'clock to remove cleaning patches. Threaded holes along the top of this tube for the installation of sight bases will be allowed.
