>>63986982come to think of it, the 57mm calibre probably is related to the British 6lber which was in use as recently as WW2
>>63986864it's a tradeoff between these key factors:
lethality, which is most influenced by weight (whether of HE filler or penetrator design)
loading machinery size and weight, which is mainly influenced by both volume and weight of shell
desired rate of fire, which is mainly influenced by loading machinery
and
ballistics, because different shapes, diameters, lengths and weights can perform differently at different ranges and trajectories
if that sounds like very very VERY many combinations of shell design can be cooked up with all these factors, well, YES, that's why it's a long process to find an "optimal" shell, and many users just buy what's available
certain special considerations include provisions for emergency manual reloading