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This is the "Redeemer" AKA "Manning's Pipe-Gun MK3" the newest model of the world's first 3d printable Shotgun, designed by Chad Manning, following in the footsteps of "The Liberator" made by Cody Wilson/Defense Distributed.
An US AFT-compliant firearm based on the Filipino Guerrilla Gun / Four-Winds Shotgun, boasting a slap-fire mechanism for a 12 gauge buckshot round. Further improvements include more ergonomic grips for easier slap-fire.
Simply get Shapeways/Download the file to print your own Redeemer, and load like you would a four-winds shotgun. (there are numerous videos out there)
This is perfectly legal, because the 2 parts are not attached 'assembled' until after the paper-work's done, you are just getting the parts made and shipped to you in the same way as if you'd buy pipes, nuts and bolts to make a Four-Winds Shotgun.
NB: This is merely a proof of Concept. I am not ballsy enough to test it myself, but I'd expect that stainless steel would be sufficient to handle IRL use.