I've always thought Doc Brown's rifle from back to the future 3 was a good example of a pseudo-primitive firearm should look like. Obviously it's not really "STEAMPUNK" but the premise is basically the same.

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It's clearly based off of a real rifle from that period, but uses brass components to achieve "the look". It also has an elegant looking scope that still ultimately appears to be designed with function in mind (though it does take artistic license).