During a high school physics experiment, my lab group created a pneumatic potato cannon to demonstrate fluid properties.
While testing the device, I was always thinking about making more accurate projectiles. My first thought was to rifle the barrel of the cannon but this would be a difficult task. Instead, making a projectile that could rotate using the air flow over its surface was the path forward.
This design allows the projectile to be loaded into a cylindrical barrel and, after being launched, deploys fins that induce a spin into the projectile.
A straightened paper clip was used as the hinge for the fins of the round. Originally, the hinge was part of the fin but the strength of that method came into question.
There are captive torsion springs in the bottom of each fin that provide the outward driving force for the fins.
When folded, the fins are tucked into the main body of the round.
Even when fully collapsed, there is plenty of internal space for additional payloads (e.g. parachute).
If a timed parachute release system were desired, an interesting challenge would be to create a fully mechanical trigger system to avoid the need for electronics within the round.