An advance is made in the art according to an aspect of the present invention disclosure directed to a pressure-activated, inertially locking base and fin mechanism for projectiles. Operationally, a base housing of a projectile contains both the fins, held within slots within the base, and a plunger. Which plunger has plurality of channels communicating with a chamber within base and which plunger is initially situated flush with the base. Upon firing the chamber is pressurized by the high-pressure firing gases being forced through the channels. When the projectile exits the barrel, the chamber's pressure vents through the channels, but, not fast enough to avoid a pressure differential that forces the plunger out of the base, causing a set of spring arms to flex upward and deploy the fins. When fully deployed, the fins are locked in place, in the deployed position, by a spring loaded pin.