A projectile has filler material placed between an outer surface of its fuselage, and fins that are hingedly coupled to the fuselage. The filler material fills space that otherwise would be occupied by pressurized gases. Such pressurized gases could cause undesired outward force against the projectile fins during launch of the projectile from a launch tube or gun, such as when pressure outside the fins is suddenly removed, as in when the projectile passes a muzzle brake in the launch tube. The filler material may be any of a variety of lightweight solid materials, such as suitable plastics or closed cell foams. The filler material prevents pressurized gases from entering at least some of the space between the fins and the outer fuselage surface. When the fins deploy after the projectile emerges from the launch tube the filler material pieces fall away harmlessly.