Patriot, I missed your post over there. Never a bad idea to have wall thickness checked. In my experience, it's unusual for a Belgian gun to be underbored like that. That should indicate more rather than less wall thickness, assuming it's not marked as a 13. As for what you shoot in it, the main issue is low pressure in a vintage gun. The length of the shell is not the critical part, as long as you keep the pressure down. You can do that either by reloading 2 3/4" hulls to low pressure or buying factory short shells. Some people report reduced felt recoil from longer forcing cones, and Sherman Bell did report some pressure reduction (although not a lot) from longer cones, when he tested long shells in a short-chambered gun--although lower pressure doesn't automatically equate to reduced recoil.
