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The bottom tube is clearly a magazine. Looks just like the bb mag I had on a shitty old "GET OFF MY PROPERTAH" air gun. (Fuck you, bird, just fuck you. I'm trying to sleep at 3am. I don't care about your goddamn tree.)
It looks like a grenade, ball, or similar round object launcher. The super-short bolt action on the back looks just big enough to shove a blank in, and BOOM! ball goes flying. Then another one feeds up from the bottom mag, waiting in a little catch pocket in that big unrifled bore.