I set up the barrel in my lathe, and set the taper attachment to match the taper on the barrel. Then chuck up on the sight, and using a small boring bar I taper the ID of the sight to match the barrel and have the correct hole size at the front so it sits on the barrel at the right point. Then I polish the ID of the sight and the OD of the barrel, and using a collimater to make sure it's lined up,I soft solder the sight onto the barrel. (Tin the sight and the barrel first)
An alternate and easier way to do it, which works but is not really correct, is to turn a parallel section on the barrel to accept the sight and open up under the ramp with a file and sandpaper to fit over the barrel, and solder it on. That works fine until you want the sight removed and then the barrel looks bad with the turned section showing.
You can also taper the ID of the sight like jewelers size a ring, if you have a steel rod that matches the size and taper of your barrel by heating the sight and forging it over the rod. Then polish out the hammer marks and fit it as above.
