Hi,
I decided to mount a silver "spider" front sight that was often used on British guns. The first step is to mark the center of the barrel top just behind the muzzle. Most British fowlers have sights very near the muzzle rather than set further back. Borrowing a method described by BJ Habermehl, I lay the barrel on a flat surface with the top flat down. I slide a file under the muzzle where I want to position the sight and draw it back. That marks the exact center of the round barrel. Next I cut my spider from 0.016" dead soft fine silver sheet. I start with a 5/8" square of silver and scribe lines from each corner to find the center. I drill the center and square the hole (about 1/8" square hole) with a needle file. Then I take of lump of silver sprue from one of my casting jobs and file it into a rectangular shape to be the sight. I file the bottom to fit through the square hole in the spider. I eventually will peen over the excess on the underside to lock the sight in place. Either using files or a jeweler's saw, I shape the spider forming the legs around the scribed lines. However, as you bend the spider to the barrel, the legs will angle forward distorting the nice perpendicular cross so file the legs so that they appear to angle back a little before fitting the spider to the barrel. I place the spider on the barrel so that I can see my barrel center mark through the square hole.
I fluxed the barrel and underside of the spider and solder it in place with TIX low temperature solder. That holds it in place so that I can get a good tracing of the spider with a scribe. That is critical because that tracing and eventual mortice will form the final shape of the spider. I heat the spider to take it off and begin cutting the mortice. I outline the spider first with a small square and then remove the metal within the outline with large, small, and tiny flats. Once the metal is removed, I clean up the edges with a tiny flat and then undercut the edge with a knife edged graver. When creating the mortice, it is easy to remove too much metal under the body of the spider, creating a flat spot under the sight. You want to preserve the curvature of the barrel and avoid a flat so cut carefully. The mortice is very shallow. At the location where the peened jug on the sight will protrude on the underside of the spider, I drill a shallow hole in the barrel to accommodate it.
I fix the sight to the spider and peen it tight. I check that the spider fits into the mortice. Don't worry if it is a little smaller than the inlet because you are going to tap and expand it. I tin the mortice with small amount of TIX solder and then place the spider in the mortice. Starting with the body of the spider, I tap the silver with a small hammer to start driving into the undercut. I anchor the body first and then work down the legs. After tacking it with the hammer, I tap it with a punch to further expand it. When that is done, I heat the barrel from the inside with a mini torch to allow the solder to flow.
Then let cool, wash off excess flux, and file, sand, stone, the excess metal away leaving a nice spider sight.
I have to clean up a little solder and polish things. When the barrel is browned/blued the silver will show up very nicely. I'll leave the sight blade high and rough until I shoot the gun.
dave
