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https://en.wikipedia.org/wiki/Munition_armour This is what you're looking for. Late medieval\renaissance mass produced, partially standardized plate armor was a thing. But it had limits, and required certain types of large scale metalworking industry and technology for it. Without it, you get...

So they are insisting on having the Dark Eldar problems... without being Dark Eldar, and trying to be a military rather than pirates with OP technology.