Algodoo

This is a rifle, at least, meant to be. It works, has a catch mechanism and drops whenever you need it to!

To open the bolt, pull the grey ball left or press the left arrow key.
Then, slide a bullet in the hole that will appear and pull the lever by the trigger down.
It, most of the time, doesn't slam forward, so go there and give it a little bump.

Press enter/return to fire, and pull the bolt back again. The casing will fall out and you will be able to put a new one.

This is what happens when you try to make a recoil gun but you don't know how it works...

Oh, and the sight is adjustable! Just drag the little dark gray ball forward and you'll be set.