Russians measure calibre from the top of the grooves in rifling, Americans measure from the pits of the barrel. As artillery is rifled there is near to no difference between the shells diameter. There is near to no difference in a direct comparison as they do the same job just as well with slight differences in the prices of artillery pieces and differences in shell types, but the Americans have developed better shells.
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There is no difference between them at all. NATO measure the caliber between the pits(B), and in Russia between the hills(A).
Both calibres are for all practical purposes identical (although not interchangeable as far as I am aware).
However NATO countries try to compensate for the numerical advantage of Russia have in artillery by designing systems that used their guns more effectively.
Research trends focussed on
- increasing the range of the weapons
- improving their ease of deployment
- increasing their rate of fire
- improving terminal effects
- coordinating fires
With more research and finance available to west slowly and steady western artillery is outpacing Russian artillery.