A fin deployment apparatus for stowable fins that comprises a sub-caliber fin hub configured to be rotatably coupled to an aft portion of a projectile, the fin hub having a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins. Each fin in a stowed position is disposed upon a respective exterior surface region of the fin hub and a respective exterior cavity or recess of the projectile. Each fin is urged toward a deployed position by a respective torsion spring, which operates to deploy the fin after the projectile has left the barrel or tube. The sub-caliber fin hub is further configured to be keyed to the aft portion of the projectile during storage and while within the bore of the barrel or tube in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody