Recently, there were some questions about making and installing underlugs in a rifle barrel. I have this series of pictures, so have included them in a short tutorial. Hope it is of use.
I will comment on what is happening in the pictures in due coarse...have to run right now.
I cut a strip of ~.03125 steel or brass using the top of my vise as a guide. The strip in this case is about 3/4" wide, inside the vise jaws.
Then I cut off pieces about an inch long, and rough polish the inside for silver soldering.
Here the piece is folded in half...
...schmeared with flux...
...and then squeezed shut.
I place the folded side in the vise between 1/8" and 1/4", depending upon how deep I want the lug, and use a drift, in this case a screw driver, to tap them open onto the top of the vise jaws. I finish the fold with light taps of a hammer.
Here the lug is hard soldered on a fire brick using a propane torch. Dull red does it.
The extra silver solder is filed off.
I've used steel ink to help lay out where I want the rectangular holes. This rifle was getting a very slender barrel, so I laid out the holes to place the key about half way down the forend from the barrel channel. Here's where your drawing is important, for that planning ahead. Often, the slot will be level with the barrel once in the dovetail. Also, this one was getting a key 3/32" thick.
Holes are drilled on the centre punch marks. I make 'em a few 'thou' under the thickness of the key to allow for filing out the web. Also, I only needed two lugs for this project, but it doesn't take much more time to make a spare or extra for a future project.
A jeweler's saw is used to cut out the steel between the holes.
...sawing is done.
...filing the slots both in length and width. I ground down a nice coarse file to a thin 'needle' file.
the slots are filed, as well as the tops of the folded metal, just to square them up and make them pretty.
More steel ink, and the tabs are cut off ready for cutting the dovetailed edges.
This shot shows the lugs set back into the vise, and the triangular safe-sided file I used to cut the dovetails.
Sliding the file over the vise jaws, cutting the ends of the lugs to 120 degrees.
I've used the Vernier's calipers to measure the thickness of the metal that will go into the barrel, locked it, and with the depth gauge end, marked the barrel for the full depth of the dovetail. Just hold the tool vertical and slide it along the bottom flat (turned down-side-up in the vise). A felt pen will also do for layout ink.
The big black mark indicates where the centre of the lug is to go, and the scratches either side are the minor dimensions of the male dovetail on the lug.
I use a hack saw to make a series of cuts just short of the scribed depth dines. Use a new blade, don't rock it, and go slowly, checking almost every fine stroke to ensure that you are not outside the lines. Here's likely where a novice is going to freak out. It's not magic - it's mechanics - and you just have to be calm and patient, and go slowly.
I usually use the hack saw tipped at about fifty or sixty degrees to cut away the webs left by the hack saw. You can also use a cold chisel with a good sharp grind, or file them out. Again, go slowly and stay inside the lines. Filing without rocking, and level, is something that comes with practice.
After I've squared the corners almost to my lay out lines, with a parallel pillar file, I use the same three sided file to create the female dovetail cuts. Stop often, and check with your lug to make sure it isn't oversized. I like to include a very small taper in the dovetail so that once started, I can go very very slowly to get a perfect fit.
...almost there. This is not the time to stop for a beer.
The lug is tapped in. I use a drift I ground from an old nail set. It has a rectangular end that is nice and square so I don't miss and dimple the barrel. Also, the extra metal has been filed off flush with the angle flats of the barrel. It probably isn't necessary but looks nicer to my eye, and doesn't need extra inletting into the channel.
Now it's time for a Canadian...or a Budweiser!
I've included another underlug picture, this one from a longrifle I'm currently building for Leatherbelly. I used brass of 1/32" thickness here, and pins that are finishing nails .073" dia. The pins pass through the stock's web right next to the barrel, and their holes are elongated just a bit with a small needle file. Here, we don't have humidity issues, and I've never experienced stock lengthening or swelling until I brought one of my rifles to Dixon's Fair. But I don't take a chance. The pins or keys sure can't go 'fore and 'aft in the wood, so if it changes length, the elongated slot will make the difference between going merrily along, or having to repair a split forend.
