Mike,
With paper, you need a type of cutter that presses/cuts or shears through from the outside, normal to the surface of the workpiece. I hope that makes sense. The old-fashioned crank cutters from a century ago work best, as they were made for the chore. They turn the shell over a supporting mandrel and have a sharp-edged rotating disc that cuts against the case wall. A few laps, and the cut is done. These are hard to find nowadays, and people want a mint for them. They work better than anything else for plastic hulls too. I found an old Lyman product which uses the same principle, but is simpler in construction. It has a cutting wheel that looks a lot like a slightly sharper version of the one on one of those little copper tubing cutters. It has interchangeable mandrels for 20, 16 and 12 gauge, and the mandrels are held in place to the body of the unit with a thumb-screw to allow adjustment of the length of cut. The cutter wheel is mounted on a spring-loaded hinge that pivots on the part that holds the mandrel. You slip a hull onto the mandrel, squeeze the cutter-wheel holder down onto the hull, and then turn the hull until the cut has been completed. Pretty slick. I hope this description helps. Before I found this tool, I contemplated making one. I still do. The only tough piece to make (not that difficult either) is the cutting wheel. frankly I think you could rob one from a pipe cutter and sharpen it a little. You can get replacement cutting wheels for good tubing cutters.
Alternatively, you could modify the tool sold by Ballistic Products. It is made for 12 gauge, and you'd have to convert it for 16 (turn down the mandrel or make a new one). It uses disposable utility knife blades, which is nice. They do not use the rotating disc principle, so the blades won't last as long, and it will take more effort to cut the hull, but it'll do the job.
Tony Lowe