Interesting, Grenadier.
There was also a .405 diameter bullet used in the .400 Purdey and some other .400s, but these haven't stood the test of time in the same way that some of the others have. I suppose the 'gap' has partially been filled not by diameter but with the introduction of 350gr .375" bullets.
(also, think you missed the .318 in between the .311 and .323)