deployed by combustion gas pressure, or by pneumatic or hydraulic forces

Full Title

Means for influencing, e.g. improving, the aerodynamic properties of projectiles or missiles; Arrangements on projectiles or missiles for stabilising, steering, range-reducing, range-increasing or fall-retarding (F42B6/00 takes precedence) > Stabilising arrangements > using fins spread or deployed after launch, e.g. after leaving the barrel > deployed by combustion gas pressure, or by pneumatic or hydraulic forces