This is a next logical step from having made the underlugs., and happens to be on the same rifle. This little rook rifle has two barrels - a .32 and a .40 cal. naturally, the lugs have to be the same and in the same place on the barrels. The drawing I made prior to lifting a file or chisel is the key...from it I can see that every part will work with its neighbour.
So, you've made your underlug(s) and have inlet them down into the barrel channel. Measure from the top flat of the barrel to the top of the opening in the lug, for the key, and mark it on the side of the stock. That's easy to say, and it may require a tutorial all of its own. Mark it on both sides of the stock. Now draw a fine line under the first to indicate the thickness of the key. Also mark the 'fore and aft' vertical lines indicating the width of the key, on both sides.
Prick punch three locations within the rectangle, and drill with an undersized drill bit half way through the stock. You may feel the bit touch the lug, as the barrel is firmly in the channel, in the drill press vise. Don't be tempted to go all the way through and out the other side, because I can almost guarantee you, you won't be in the rectangle you've drawn on the other side. Turn the stock over and repeat with three holes on the other side. (for each key)
Now a little something about tools. I made my chisels for this job by grinding down an old file or two. In this case, I have used a file for sharpening spade drills. It is already thin, and once shaped like the middle one, cuts these rectangular holes very easily. The one on the far right is about 3/32 thick, and I use it for fat keys, trigger slots, and barrel tennon inlets.
This skinny little chisel is inserted into one of the outside holes, bevel to the middle as in the photo, and is used to break out the webs between the holes. Keeping the long side of the chisel to the outside of the rectangular hole prevents enlarging the hole by not crushing wood you want to keep. Do the same on the other side of the stock.
This shot shows a piece of steel I've filed to the same dimensions as the key, but about three inches or so long. This I heat to a dull red on the last 1/2" of the tip, and push it into the wood to burn the slot to exactly the size of the keys. Smoke will billow out of the hole, and the heated tool will come to a stop as it cools. Reheat, and go again. Continue right on through the stock. in the direction from which you intend to have your keys go into the stock. The hole on the entrance will be a little looser than the exit hole, and that's good for the key. When you do this on a stock that still has wood to remove, that charred entrance is cut away, and you are left with a clean sharp perfect inlet. Even on a precarve, a little filing and scraping will clean it up nicely. I took these pictures after I have shaped the forend, just to show the process. You can see that this system will work even without the use of escutcheon plates, as the keys fit without gaps or chips.
Here's a picture of the key in the new hole. Piece of cake.
The next few shots just show the finishing stages of the forend...the fun part. When you compare the profile shot of the unfinished rifle to the drawing in the second picture, you can see I didn't deviate to far from my original concept.
