Sheepherder... another method for which you
ARE tooled that will work as long as you use CLOSE tolerances.
Machine the barrel just as you have illustrated and measure it carefully.
Then bore out the front sight band to EXACTLY (or as close as you can) to the SAME dimension.
Mill a TINY witness mark/groove/keyway on the bottom of the barrel, that you have properly indexed to be the bottom of the barrel about the diameter of a straight pin, and very short! Length is not important here, only position.
Then mill a matching groove in the bottom back of the front sight band that you bored out.
Cut a piece of wire/straight pin/etc. to fit carefully in the groove you cut in the bottom of the barrel. This will be your index so you can't get the front side band on at an angle. Solder/superglue that little piece into the groove on the barrel. Then put the barrel in your freezer for about 20 minutes.
When the freeze is over, mount the barrel in a vise muzzle up.
Quickly heat the front sight band with a propane torch just until it starts to barely glow---pick it up (with tongs or pliers, not your fingers) and position it on the barrel using the pin/index as your guide. GET THIS RIGHT cause you only get ONE SHOT AT IT. (No pun was intended, but it works in this case
)
Let it cool at room temperature, and I would wager a small sum that once cooled in place you won't shoot it loose.
This interference fit method was engineered by BMW to hold the rear wheel bearing on my motorcycle, and that bearing supports the weight of the entire motorcycle, and all the G-forces of the rear wheel---- it won't pull it off once cooled.
You could of course practice on a piece of scrap eh?