The way I do it is to place the barrel on the kitchen counter muzzel towards the back splash. put the scope base on the barrel in position, lay somehting long and straight like a pencil across the base and level it by sighting across it looking at the top of the backsplash. When it is level and in the right position, drip a drop of super glue in a couple of holes and let it set. Then you can select a drill that just clears the hole in the base, use it to mark the spot a few thousanths deep, tap off the base and drill with your tap size drill. Good luck. Larry