The screws I got from Galazans had the slot cut deep enough, I was stuck w/it's location on the screw. So, I chucked them in a drill and lightly filed the bottom of the head until the slot was orientated correctly when the screw was tightened all the way down. Then filed/sanded on the chuck from the top until flush. Takes a lot of time, especially when you're getting close. But, I'm no machinist or gunsmith.
Lots easier on a gun that's been annealed heading off to get case hardened. Everything from the top can be done in the gun without messing up any finishes.