I have yet to see a CORRECTLY installed muzzle brake
make a rifle shoot worse then before installation. There are several reasons why this happens.
1. Generally, especially with factory barrels, the muzzle crown is ALWAYS off axis to the bore, recutting the crown to match specs is probably the number one reason a braked rifle shoots better.
2. The weight of a muzzle brake changes the harmonics of a barrel. It seems that any weight positioned out past the muzzle has a positive effect on a barrels accuracy, likely because it dampens or at least makes barrel vibration more consistant while the bullet is in the bore. I am not saying the effects of the muzzle brake will improve accuracy, simply that its weight at the end of the barrel has tends to be a positive to consistancy.
3. Dramatic reduction in felt recoil which allows the shooter to relax and be more consistant from shot to shot!!! While the other two will have more effect on the rifles accuracy and consistancy performance, this aspect is what makes the shooter pilot the rifle more consistantly and for that reason, this is generally why you see better results down range above all other even though it has very little if any effect on the rifles pure accuracy potential.
Now, that is for a brake that is installed CORRECTLY. I have seen many a rifle that had their consistancy destroyed by a poorly installed muzzle brake.