Dear ccdjg - I think we are getting nearer to truth for .22 if we consider development of these calibers for airguns as an accidental progression from the likely tooling base that the introduction of rimfire made available. Basically, if you have the machinery to make the barrel for one, then you can adapt and make the barrel for the other.

If so: do we have any evidence of 'identical' barrels for both air rifle and rimfire to mark the first 'prototype'?

....which then still leaves us with the oddity of .177. Perhaps six of one and half a dozen of the other?.

OR Does this mean Havilland & Gunn are the first makers of .177?