Ordinance Identification

[Flying10uk]

A 6" naval gun's diameter is 152mm.

Are you sure the "152mm" hasn't been rounded down to the nearest whole figure from the actual metric equivalent of 152.4mm (6"), a common practice?

It sure looks like the 155mm I had...

Are you both saying that a 155mm shell needs 2.6mm/3mm clearance between it an the barrel? It seems a lot of clearance. Yes there are the drive bands but don't forget about the gaps in the rifling to fill/seal.