On the barrel , above each tenon , mark a line where the center of each tenon is on the barrel. Next , put the barrel back in the stock , and continue the center line from the barrel onto the wood. Measure the diameter of the barrel at the tenon mark , and set that measurement on your 6" adjustable square. Put the body of the square on the top flat of the barrel , with the preset barrel diameter against the center line of the pinning loop , and mark the bottom of the barrel on the loop center line. This is the place the pin hole goes through the pin loop. It's best to mark the wood surface pin hole spot , with an ice pick so the drill bit doesn't skid from true center. Works for straight barrels as well as tapered and flared barrels.
Hopefully,you have a vise and can hold the stock with barrel in it , so the top flat can be leveled . If your hand held electric drill , has a level built into the body of the drill motor , the gun barrel flat is held level in the vise , just level the drill motor ,with the bit in the marked hole and drill through the stock , and pin loop and on to the other side of the stock. Also , make sure to drill at a right angle to the barrel so the pin is straight with the longitudinal aspect of the barrel , at 90 degrees.. Used this procedure for about 50 yrs. , w/success. No drill press device setups , with special jigs,etc. Just measuring , marking , and drilling.........Simple , like done in the days before power tools..................oldwood