if the housing where the bullet fires can't be plastic because it would explode/be unreliable past a number of shots
a shell casing made out of plastic ain't gonna be able to contain a closed explosion of fired gunpowder
and there would have to be some metal involved to spark the gunpowder
its unrealistic
go break a tree branch off a tree, fireharden it and tie a string to it. now you have a bow and arrow that is 9 billion times more reliable than a 3d plastic gun
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