The Sidewinder is a rather unusual submachine gun designer by Sidney McQueen in the mid 1960s. His objective was to create a weapon which could be fired one-handed with the firepower of a submachine gun rather than a pistol. What McQueen came up with was a good example of the “arm pistol” concept, which can also bee seen in the original Bushmaster weapon of this type.
The magazine well of the gun could rotate independently of the grip, allowing the shooter to hold the gun with the magazine canted inside the arm and the buttplate resting against the elbow/bicep area (although some of the guns were made with extendable buttstocks that would be shouldered as well). In total it appears that about 20 Sidewinders were made, in three different models. The SS-1 was chambered in 9mm Parabellum, the SS-2 in .45 ACP, and the SS-3 had interchangeable barrels and a bolt with a 9mm face on one end and a .45 face on the other, allowing it to be flipped around and used with either cartridge.
Development took place from 1966-68, and the idea stagnated for lack of capital and interest at that time. Interest picked back up in the late 1970s, allegedly because of interest from Special Forces units in the military looking for a weapon that could be used while roping out of helicopters. Whatever the reason, the guns were actually manufactured in the late 70s. Full production was anticipated for 1980, but that failed to materialize as no military contract was ever obtained.
Mechanically, the guns are blowback operated from an open bolt, with progressive triggers – a short pull fires in single shot mode and a full press fires in fully automatic. The guns were fed from Sten (9mm) and M3 Grease Gun (.45) magazines. The 9mm SS-1 weighed in at 4.7 (2.1kg) pounds empty, with a 9 inch (221mm) barrel. They were designed with basic open sights, and provisions to mount the high-tech optics of the day, like Armson OEGs and Weaver Qwik-Points (which you will see mounted on this one in the photos above).
Thanks to Joe, we have a series of photos of gun #18 (model SS-1) disassembled: