Abstract: Based on results of fire tests on heavy and small calibre guns, It has been found that (1 ) jump angle changes with the range， and (2 ) magnitude of the test jump angle changes with target distance and is approximately proportional to the range of angle of attact of the projectile. The article gives the results of a theoretical analysis showing that the traditional method of jump angle determination is defective.A new concept and definition of jump angle is given, and it is advised that jump angle determination be reconsidered in order to improve the accuracy of the firing tables and to improve performence of the weapon system in firing。