According the existed issues that reference rounds speed index test point and practical application is disagreement in speed ballistic gun, research on velocity conversion method of speed ballistic gun in different speed point. According to the relevant standards ballistic gun, checking the speed of different speed point conversion formula is given speed trajectory points the gun at different speed rate conversion method and conversion formula, taking a new 12.7 mm shells species as example, given its speed conversion formula. Experimental results show the method can obtain standard’s different speed rate conversion formula.