There’s some interesting information over at Gwinn Firearms about this 1970s conversion of the M16 rifle by Bushmaster, which was tested by the Air Force to give downed pilots a little more punch and range than a ordinary handgun can afford. Component diagram:

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Additionally, here’s a video link I found, which shows the pistol in operation. […]

The cover of the June 1981 issue of Future life. There’s something about a flimsy NBC suit, the triangular handguard version of M16 and Ray Bans that takes you back to the world of imagined postapocalyptic adventure.