Well, it is not really a gun, but since it fires things that hurt, I figured I would include it to the list.
The Ballistic Knife was developed for Soviet Special Forces as a weapon for use when the report of a gunshot was undesirable, like when neutralizing a sentry. They were issued to the Spetznaz in large numbers in the 1980s.
The knife works by using a powerful spring to launch the blade about 20 feet. When combined with the standard, overhand knife throwing motion, the power behind the weapon is very great and would likely pierce soft body armor.
To fire, one would remove the pin (like on a grenade) and depress the firing button.
Another weird gun, another idea that didn't really make it.