I just had a thought.
Okay, so the story I linked above describes a projectile being loaded and fired; one of the things it mentions is that the propellant is a gel-like material.
What if the number figure on a gun doesn't refer to muzzle caliber, but to the amount of propellant gel filled into the casing? Lower-number weapons would use less propellant, making the shells lighter and faster to reload; larger caliber ones use a significantly more filled cartridge, resulting in a more powerful shot but also a longer recoil compensation time and slower loading process.
It'd also explain the whole one-shell-for-many-calibers thing: Different turret turrets have loading and recoil compensation equipment calibrated to work with shells of a very specific mass and balance. Load a shell with to little propellant, it might be flung out of alignment by a loading arm designed for a much heavier shell; load one with to much propellant, and the explosive force might damage your breech. In the time it takes for our weapons to reload ingame, the shells could be being drained or refilled of propellant gel to fit the correct weapon profile.
Thoughts? Retarded, genius, somewhere in between?