User blog comment:Eles99/Study of Shelling Critical Hit/@comment-114.79.55.55-20140508175126

maybe just maybe...

people mistook armor negating effect of armor piercing bullet with increased critical chance, since any damage above 39 is critical attack

i don't have any proof that type 91 shell negate armor, i'm just guessing from the name

and since this game have hidden stat maybe equipment library description hold some hint(s) about hidden stat