I use a 10" hardwood dowel and disposable Xacto Knife blades to trim shells. Place dowel in shell until it bottoms out. Mark dowel at shell top. Measure amount below that mark the amount shell needs trimming. At that mark, drill small hole perpendicular to dowel shaft. Insert point of blade into hole and hold blade in place with thumb and dowel with the same hand. With other hand twist shell on dowel, cutting shell as the shell is turned. When it bottoms out, it's done. If you need a photo, I'll take one latter today and post.
