1) mark the barrel where you intend to drill.
2) measure across the flats with a Vernier calipers and write down the dimension.
3) slug the barrel with an oversized lead ball, ie: a .530" ball for a .50 cal barrel, and measure the largest diameter. That's your groove depth.
4) subtract the groove dimension from the across the flats dimension. Divide the remainder by two. That's your barrel thickness at that point.
5) determine how deeply into the barrel you intend to drill...suggest leaving .040" or so between the bottom of the hole and the groove depth.
6) select a number drill that is close to that dimension.
7) clamp the barrel in your drill press vise and install the tap drill into the chuck. Adjust the vise so that the tap drill will be on your mark, lock the table and clamp down the vise. Without turning on the drill press, bring the drill down until it touches the barrel, place the number drill depth of your hole) against the quill gauge and bring the lock nuts down until they contact the drill bit. Tighten the lock nuts.
Now you have the press set up to drill only as deeply into the barrel as the thickness of the number drill. It will stop against the lock nuts drilling only the diameter of the number drill you selected.
Did I miss anything?
