19 are retained in the positions shown in fig1 by means of a retainer and release mechanism designated generally by reference numeral 30 the retainer and release mechanism 30 comprises a fin latch 31 a retainer plate 32 and a pin 34 interconnecting the fin latch 31 and retainer plate 32 the pin 34 is provided with a reduced section 35 which is designed to fail in tension due to the forces of gun launch while pocessing sufficient strength to enable the mechanism 30 to hold the fins 19 in the position shown in fig1 during normal handling and stowage referring now to fig2 and 3 it can be seen that the fin latch 31 is of generally cylindrical configuration and is provided with a plurality of circumferentially spaced notches 36 the notches 36 define a plurality of lobes 38 which are shaped to conform to corresponding notches 39 formed in the fins 19 each lobe 38 has an inner surface 40 inclined at an angle a to the central axis of the fin latch 31 and each notch 39 is provided with a surface 41 inclined at the same angle a to the longitudinal axis of the fin 19 when the retainer and release mechanism 30 is fixed to the fins 19 in the position shown in fig1 the surfaces 40 and 41 define a plurality of inclined plane mating surfaces sloping inwardly in the forward direction toward the projectile axis the purpose of this configuration will be descirbed subsequently in order that a better understanding of the invention might be had its mode of operation will now be described the projectile 11 is loaded into.