Well Taylor, I hadn't really planned on showing this little trick, but you just got it out of me.
This tool is called the "Piloted Abrasive Face Mill". We drill through a piece of flat wood with our well squared drill press, glue in a dowel of suitable size, adjust the pilot diameter if we have to, punch or cut a hole in a piece of adhesive backed sand paper, stick the paper on, and there you go!
I buy adhesive backed sanding discs to cut up and stick to this and Popsicle sticks and dowel rods. You could just use glue and regular sand paper, but this can be peeled off and replaced.
This one was used while fitting a breech plug, for crown work I would make the pilot longer.
In use, the mill does not do most of the metal removal, in stead it generates a shiney high spot which gets filed down a little, just like stain transfer work. When it gives an even shine all the way around, the end is square.