u/JMHSrowing

Weight of shell is one of the most important factors in a naval gun of the earlier 20th century, especially needing to consider what a can be handled by sailors due to less machinery assistance than would be found later in the century. It would seem to me though that 150 lbs, equating to about a 170mm or 6.7" gun, would be one of the optimal calibers. One strong man's maximum to handle repeatedly with some ease is about 75lbs, and this can be seen in naval shells and cartridges like the 5.5" which had an 80lbs shell for that reason or the 5"/25 who had an 80lbs cartridge. These guns were able to have a good rate of fire seemingly proving the idea. Thus it would seem to stand to reason that to have the maximum effectiveness of a gun which with a shell two men could handle the shell of that it would be 150lbs or maybe slightly more.

It would also be the largest reasonable for a separate cartridge system before automatic weapons as the powder and casing should weigh in the neighborhood of 80lbs so could be handled by a single man.

In addition to it being of a good weight to achieve a good rate of fire, being about 1.5x the weight of a standard 6" shell would give the ships a ballistic advantage over light cruisers so armed. As cruiser armor often only really gave protection from 6" shell, it would seem this would likely be the minimum to be fairly confident at being able to overwhelm one's foes.

Pictured is a 17cm SK L/40, one of the few naval weapons which had about this shell weight (lighter trending German shells meant it was 141lbs), spending its second life as part of the Atlantic Wall.

Was the lack of use if weapons in this caliber simply inertia with the long history of the 6"/155m and then the naval treaty limit of 8"? Or was something like this really slightly too big and the 130lbs heavy 6" shell really the reasonable max for a high rate of fire naval gun?