They just look better. They're more fun to use. They don't have useless pointy parts.
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Different design ethic. The Y1 and Y2 weapons are modern weapons that just have a paint scheme that is IB/Iron Wolves themed. The Y3 weapons are designed to match the "Dark Ages Nordic warrior" theme of the Iron Lords and Iron Banner gear for the year. So the idea is to create a firearm....that still looks like something Conan the Barbarian would use.