Dennis...........there is an easier way to do what you just explained. Pre-drill the underrib with a clearance drill, also drill
the countersink hole for the head of the screw. Clamp the rib in place on the barrel using two of those little steel parallel
clamps (couldn't build a gun without them). Then, hold the barrel in your mill vise, put the tap drill in your chuck and line
up each hole, one at a time, and drill the hole. Then replace the drill with the tap. I found what when the tap bottoms
out, it will just slip in the chuck.....so easy, have done so many that way..............Don