Same theory as with the standing breech on a break open gun. Early designs had a sharp corner, it was soon discovered that adding a slight radius, a significant amount of strength was gained and it reduced the incidents of cracks forming at that point. I certainly agree with you on that point LRF. I have been toying with building a Gibbs action and added the radius to my design as well. I do think the round block Webleys actually hold up better than the model projects.

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