Jayne's large rifle in Serenity
One might not think such a prop could be obscure given the general fandom devoted to this film and especially to the character's Firefly
counterpart "Vera," but aside from brief mentions of Adam Baldwin naming the rifle "Lux" as a posthumous tribute to a fan, it seems to have received practically zero interest... which is so weird to me, because it's my
very favorite sci-fi gun.
My understanding is that it was constructed at ISS by modifying a standard assault rifle from 1998's Soldier
(itself built around an AKU-94 bullpup) and adding a gas-powered barrel, lights, etc. Unfortunately, I've confirmed from multiple sources at the company that it was long-ago disassembled, and efforts to track down either the removed parts or behind-the-scenes photo documentation have come up short.
As far as visual reference, below is all I've been able to find:
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