Miller,
I agree with you about someone with a vested interest carrying out testing. Hard to be objective even if they want to be.
My take on the 21 being "over designed" is that; given the materials used, it has features like the barrel wallthickness, reciever bar, reciever breach face (and supporting structure), and locking system, etc., that exceed what is necessary for the ammunition it was designed to shoot. I think most guns have "over design" to some degree or another. The 21 seems to have more margin than most other guns.
My take on rotary bolt designs is that they are placed with a mechanical advantage over underlug designs. However, they rely on typically less substantial structure on the barrels to hold on to (rib extentions). The rotary bolts tend to have small cross sections in shear compared to underlugs.
So, sizing of the parts and materials used would seem to be what makes a particular one stronger than another.
