John, I found the same problem. On the dowel type I have in my basement shop I've added a machine screw in the end so it can be adjusted for length. That usually means a couple of wrong cuts and discarded shells. Your idea ends that problem. The dowel rod with razor is still good for paper shells because it supports the paper as you cut. For anyone without a band saw, your idea is the best I've seen so far for plastic shells.
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Paul Harm
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