[Zeb[edee]]

Perhaps Games Workshop fell asleep during biology? Thats my two cent. Or about one penny at the current exchange rate.

Either way, I think that the swept back design would be a wind resistance thing? Their guns are pretty big and powerful and would cause a recoil so perhaps they are trying to reduce the amount of effect that wind has on the movement combined with the recoil? So perhaps it is not a physiology thing but an adaptation to keep their aim true?

I would imagine that being aliens the differences between prey and predator would be quite different. Perhaps the prey is the fauna on the planet and every animal is a predator? Just spit balling here.