The initial problem is energy and the air.
How would a mechanism like this work? Pressurised air inside a cylinder that is released when a projectile is detected?
Instead of pressurised air inside a cylinder, could it be an explosion? An explosion inside a tube? (This kind of defence is already used in tanks, but they shoot projectiles to fragment or detonate the projectile once the reactive armor is struck).
A turbine (a plane turbine or a rocket turbine) that would increase the turbulence in the air to increase the chances that the projectiles deviate from their trajectory?
Or would just be better to use electromagnetism to repel the metallic projectiles?
Projectiles could be anything from a missile to a bullet.