I put some tape on the stock and used a pair of calipers to take measurements off of the relief hole on the right side and one of the letters on the left side of the gun. Marked on the tape and drilled a very small pilot/test hole to make sure it was centered right. Put a little play dough or something soft in the set screw and then push the small bit into the hole you drilled. It will leave a mark in the play dough. If centered, just run the correct size drill bit.
I don't know if it was my new bits. But the carbon/magnesium really wanted to grab the bit and turn it into a screw and suck it down quick. Keeping the speed of the bit up, it drilled very nicely.