[Berkeley492]

I have (2) 1851 Navy squarebacks. At the muzzle mic'ing is 0.630" on one and 0.640" on the other. At the breech end one is 0.797" and the other is 0.793". Both have had some degree of 'cleaning' in the past which would account for the small dimensional differences.
I do have a few other later Navies (1860's vintages) that I could check, but you would need to find a really pristine original to determine these dimensions precisely.