Abstract: A guide mechanism for a projectile (2) which is intended to be fired from a barrel (3) with the aid of propellant gases. The projectile comprises a plurality of elongated fins (5) which are journalled at one end thereof and which are arranged to occupy a folded position during passage of the projectile through the barrel. The fins are subjected to a fin-extending force with the aid of a piston (7) which is arranged for movement in a housing (6) in the guide mechanism. The guide mechanism also incorporates a channel means (10) through which during a pressure increase phase of the propellant gases in the barrel the gases are passed to a chamber (9) located on one side of the piston and build-up a piston working pressure (p) in said chamber. The channel means (10) has arranged therein a valve (11) which is operative in throttling the channel means during a pressure reduction phase of the propellant gases.