How do we keep the hardness intact in the file heat of soldering will damage hardness........
Allen, the files only have to be a little harder than the golf ball, so no amount of annealing will make the files softer than that. Have another cup of coffee!
The whole bs paragraph on the files was of course just that, since some were worried that sandpaper wouldn't be perfect. For the time you'd have to put into the file-dovetail-slot cannon or the soldered variety, you could easily make dozens of sandpaper-equipped tubes and do one shot in each so no worry about reloading ever.
Be sure to put black stripes around the golfball, at least two at 90 degrees apart, and have a suitable high-speed movie camera set up to record it, so you can easily tell the speed and direction of rotation on muzzle exit.
And report your results here, of course.