I didn't post instructions because it is fairly simple and you can make your own easily in your own way. It was first a rifle-like weapon and was a simplier design, but than i changed it to a pistol. You can built a rifle version in the beginning for simplicity, in which the trigger(knex rod) is in backside of the gun.
It is fully made of Lego except a few k'nex parts. It could also be built fully with knex but i prefered lego. I have posted some images with different angles to make the mechanism clear.
Warning: Don't use anything other than paper as bullet since the gun's upper side is open, there is a little risk that it can backfire. Also it can be harmful for eyes in close range, don't shoot at people.
Step 1: Paper Bullet
Step 2: Mechanism
Step 3: Mechanism
Step 4: And Finally Aiming
3-Where the cock sits
4-Rubber band holder in the very front
There is not any other complex part in this gun. The other parts are just for holding the gun together.
note: you can build a rifle-like version, by using the lover side of the cock as the trigger. It is much simpler and you can convert it to a pistol is you want. Only the rifle-like version requires two hands to use.