Download video: Single Shot Firearm Breech Block Design
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Here's a really fascinating WIKI page about "bolt thrust" or "breech pressure": https://en.wikipedia.org/wiki/Bolt_thrust
If accurate, the article quantifies the typical force applied to a breech face by different rounds. Gives you an idea of how much strength needs to be designed into a breech.
Here's a .pdf file of average shear strengths of different materials: http://www.freetechnicalcharts.com/media/Average_Shear_Strength_of_Materials.pdf
9mm Luger rifle build video: https://youtu.be/fAiwkGaoSbQ
Break barrel 12 gauge shotgun build: https://youtu.be/grLuCKRsPek
22lr derringer pistol build: https://youtu.be/-in7EFuVLI8