A projectile and deployment method ensures successful deployment of the projectile regardless of an external environment. Contacting engagement is maintained between a piston and deployable fins as the fins rotate from a folded position to a deployed position. The fins are pushed by the piston to rotate into a deployed position in which the fins are locked before the piston is able to eject from the assembly. Using the engaging tabs between the fins and the piston, and a modular pressure reservoir, the piston continues to push on the fins at least until the fins are deployed and locked. After locking, pressure in the projectile is equalized and the piston is launched off of the pressure reservoir. If the fins are not immediately deployed and locked, the piston will continue to push on the fins until the external environment enables full deployment or until the pressure is equalized.