OK, so picture this: MacGyver is hiding in a medical lab, and needs to build a weapon using whatever's at hand. Well, if he had a syringe (along with lots of scrap aluminum), he could perhaps make himself an airsoft-type rifle … not unlike the one recently built by our friend Patrick Priebe.
The syringe acts as the rifle's compressed air chamber. When the plunger-like "piston" section of the syringe is pushed in, the air inside the syringe body is forced out through the opening on the other end, where the needle would ordinarily go. Instead of just going out into the atmosphere, however, that air forces a BB down an attached 5-mm brass barrel – at quite a fair velocity, too.
In order to attain that speed, a spring-loaded steel rod is used to force the piston into the syringe, hard and fast. Once a shot has been taken, a pump-action mechanism is used to pull that rod back for the next shot. A locking mechanism keeps the rod/piston drawn back until the gun's trigger is pulled.
The BBs are loaded through a small airtight Teflon compartment between the syringe and the barrel. And, because it's one of Priebe's cyberpunk creations, the rifle also has a red aiming laser.
More information on how the gun was made, along with footage of it in use, can be found in the video below.
Source: Laser Gadgets by Patrick Priebe