This 7" American gun looks quite nice. Originally it was used as the secondary armament of battleships, then they decided that they'd be great for ww1, but couldn't finish the project before the end of the war. It's also strange that they had a gun with a nominal 177,8mm calibre, yet in the 1950s they developed a 175mm gun. Not that making some new tooling is that hard for the American industry, it's just strange that they switched to metric. Although 6.9" is 175,26mm, so it's possible that the nominal calibre is that, and the 175mm is just rounded down. On the other hand, Americans used French 75mm and 155mm guns, so metric calibres aren't unheard of, they even developed a 90mm gun on their own. during the 1930s. It actually makes me wonder if they switched to metric over time. Of course it would be quite helpful is we knew the exact measurement between both the lands and the groves, not just a nominal value that might be rounded up or down.