Interesting post! There is yet another issue: If indeed a 2 1/2-inch chambered gun will sell at a discount over a 2 3/4-inch gun, then remember that it already sold at a similar discount when you bought it!

So you bought it for less and will sell it for less if your model is correct, which I do not think it is (but again, that's just my own opinion)

The comments regarding proof pressure of the CIP vs SAAMI loads are important. Most of us, me included, do not use commonly available domestic loads in our European guns proofed for European shells. Given that, then any difference between 2 1/2 and 2 3/4-inch shells just about disappears.