Cinders, are you able to explain how the shell can be for a 155mm gun when it only measures up as 152.4mm in diameter? To quote a precise dimension such as this (152.4mm) I assume that Biggunsarebetter has measured the shell accurately with vernier callipers or a micrometer, unless he has extremely good eyesight. I recognised the figure of 152.4mm as equating to 6" as soon as I read it but it is easy to check/convert by dividing 152.4 by 25.4 which gives 6 in this example.

The reason that I suggested naval is because it was a commonly used calibre of gun used on cruisers of the Royal Navy and Commonwealth navies during WW2 and beyond. A good example of this is HMS Belfast, now a museum ship, which was originally part of the "Town class" of cruisers and their main armaments consisted of 12X 6" guns mounted in 4 triple turrets. After hitting a magnetic mine in 1939 and nearly being lost, HMS Belfast was substantially rebuilt into a class of her own (improved), featuring many alterations and improvements but still retaining her main armament of 12X 6" guns that she has to this day.