Mark has answered your question very well. I got this method of inletting barrels - parallel, tapered and swamped - fro articles in Rifle Magazine by the late John Bivens. I used sheet rock screws because they are hardened, have a very thin section, and an aggressive thread. I drilled the holes in the 3/8" square key stock as close to the outside edge as possible and a tight clearance for the screws. I made a centre punch which just fits the holes, and after the rails are clamp0ed to the barrel which in turn is clamped to the wood, I centre punch each hole. then I use a hand drill to drill pilot holes for the scrws, wax 'em up and run 'em in with an impact driver - gently. Now I remove the clamps, lift out the barrel, and using a piece of rip saw blade with set removed from one side, and handle attached to cut on the pull, I cut down along the inside faces of the rails to the bottom of the side flat. I have two marks scratched on the saw to indicate maximum and minimum depth for a swamped barrel.
From here on, chisels do the work, as well as a Vernier's calipers to measure depth as I go. To get a perfect fit, I use up an eight hour day. So recently, I've been using the services of Mark Wheland and David Rase to inlet my barrels and drill the rod holes. Their work is super crisp and sharp, which is a great departure from the interesting charm of my own inletting.