It is a simple gun which shoots paper projectiles. It can penetrate an A4 paper from 1 meter.
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

just paper

Step 2: Mechanism

there is a balance between the two sides. When you pull the trigger, the rope gives up and releases the knex rod. And the rubber gets free

Step 3: Mechanism

you can see the details of mechanism

Step 4: And Finally Aiming

the gun is not symmetric, but in the end it doesn't matter

Step 5:

Ok here is the parts in more detail but i won't release any full instructions. There is no need to release instructions for connecting simple duplo bricks. You can build your own if you get the mechanism.

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1-Trigger
2-Cock
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.