Use a ruler and pencil on the stock to mark a line where the center of the barrel lug that you intend to drill is located, and scribe that line onto the side of the stock that is going to be drilled. Then with the barrel out of the stock, turn the barrel on one side, so that the top flat can be set against a block of wood, or metal, which will be clamped to the table of your drill press. Using a smaller drill bit in the chuck, set the block so that the drill bit is centered in the barrel lug. You can mark the lug for the final drill be turning on the drill and just touching the lug with the pilot drillbit. Finally put the barrel back in the stock, using clamps to hold it firmly in place, and set the barrel against that wooden block and find that line on your stock. Drill a test hole with the pilot to see if you hit the mark. If everything is lined up, then go ahead and change the bit to the size you need for your pin, and drill the lug and stock together.
You will want to remove the barrel and lug from the stock again, and widen the pin hole to allow for expansion back and forth in the stock. You may also want to make the hole in the lug the next sized drill bit just so you don't have any trouble running the pin into the lug hole.
Obviously the block you clamp to the table should be tall enough that you can brace and clamp the stock and barrel to it firmly for that final drilling. This is the kind of project where the set up time takes 5 times longer than actually drilling the hole. But, it is that important to do it right, so take your time, measure 3 times and cut once.
