What is the bore diameter of the PK reamed to? Should be 20 thou over bullet diameter. If its to large you can often get turbulance around the bullet. If its to tight, or was not deburred, you may be getting some bullet to brake contact.
I have never seen a rifle shoot worse after getting a PK installed compared to how the rifle shot before. They always seem to shoot at least as well as before if not better.
Was the muzzle recrowned after fitting the muzzle brake
. It should have been.