Abstract: The boat tail (28) of a fin-stabilized projectile (10) for firing from a smooth bore gun tube (14) is provided with a plurality of transversely extending spin chambers (34 a,b,c,d). Each chamber communicates with a converging-diverging nozzle (36 a,b,c,d) which extends through the outer surface of the boat tail (28) to provide free access to the surrounding atmosphere when the nozzles (36 a,b,c,d) are clear of the gun tube (14). When the propulsive charge in the rear of the gun tube (14) is ignited, a portion of the propellant gas produced by the charge enters the spin chambers (34 a,b,c,d) through the nozzles (36 a,b,c,d) and is stored in the chambers (34 a,b,c,d) until the converging-diverging nozzles (36 a,b,c,d) clear the gun tube (14), at which point the propellant gas exits through the nozzles (36 a,b,c,d) in the form of high speed tangentially directed jets which cause the projectile to spin at a predetermined rate which reduces dispersion due to fin misalignment.