I have been working with brake design for over 15 years. I even had an engineer who has super sonic wind tunnel testing experience from the aircraft industry get involved, (my Father, one question and thirty pages latter) . Bottom line is angled holes, spiraled holes and that other stuff is bunk and sales pitch!!!! As temperature, (Burning powder) goes up so does the speed of sound the higher the temp. the higher the speed of sound. however gas above the speed of sound does not like to turn corners and thus what ever angle you drill your holes in reality you are dealing with the frontal area of the holes. The frontal area is the same weather the hole is angled or not so like mentioned before do it the easy way and drill them 90 degrees to the center of the bore.