I had muzzle brakes installed on two 14" barrels. The first was a 7tcu and it shot great both before and after, just more pleasant after. The other is a 300 whisper, the brake completely changed the harmonics, but after changing powders it came right back into good accuracy.
Both barrels would shoot averages of 1" or less before and after the brakes were installed.
Both barrels were or are shot in Handgun Silhouette in the Unlimited Standing category with high rise mounts and rifle scopes. Too much fun.
Joe