SPIN DECAY PROJECTILE

Abstract: An explosive round for a rifled gun has a bullet (14) with fins (11, 12, 13) extending along the length of the bullet forward of the center of gravity. The fins decay the spin of the bullet imparted by the rifling after the bullet has exceeded its maximum accurate range. After this range, the bullet tumbles rapidly thereby limiting its length range.

Description: SPIN DECAY PROJECTILE BACKGROUND OF THE INVENTION This invention relates to an explosive round having a relatively fixed limited range, and more particularly, to a bullet having spin decay fins for limiting the range of the bullet. Ammunition has a limited range of accuracy. However, the bullet travels far beyond the accuracy range and remains lethal beyond the accuracy range. For example, service ammunition such as that for the 7.62 mm machine gun has a range of approximately 150 meters. It is desirable to provide practice ammunition which will match the trajectory of service ammunition for 100 to 400 meters, but which will have a limited maximum range. In the case of the service ammunition discussed above, the requirement is for a practice round with a maximum range which does not exceed 400 meters. The need for practice ammunition with a limited range is best pointed out by the very small number of practice ranges at which high velocity military ammunition can be fired. Because of this, training has been restricted. A limit on the maximum lethal range is also required for sporting ammunition. Rifled weapons are forbidden in many hunting areas because rifled bullets remain lethal far beyond the range of maximum accurate use. It is an object of the present invention to provide an explosive round with a bullet having a maximum effective range which is just beyond its range of accuracy. Another object of the present invention is to provide low cost practice ammunition which emulates the trajectory and accuracy of service ammunition for 100-1,000 meters. It is another object of the present invention to provide ammunition suitable for sport use where a short maximum range is critical. Summary of the Invention In. accordance with the present invention, the bullet for an explosive round has a plurality of fins extending along the length of the bullet forward of the center of gravity. After the bullet has exceeded its maximum accurate range the fins decay the spin of the bullet causing it to tumble, thereby limiting its maximum range. Fins are not normally placed on projectiles forward of their center of gravity. Fins have a stablizing effect when they extend backward from the center of gravity but it is generally thought that fins in front of the center of gravity are destablizing. However, ammunition with bullets with fins forward of the center of gravity remain stable with an accurate projectory over the range of high velocity, the range of normal accuracy. After this range of accuracy, the fins rapidly decay the spin causing the bullet to tumble and abruptly shorten the maximum range. The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and claims. Short Description of the Drawings Fig. 1 shows a bullet with three spin decay fins; Fig. 1A is an end view of the bullet of Figure 1; Fig. 2 shows a round with four spin decay fins on the bullet in accordance with the invention. Fig. 2A is an end view of the bullet of Fig. 2; and Fig. 3 shows bullet drop vs. range for various ballistic coefficients and muzzle velocity at 2,500 ft./sec; Fig. 4 shows bullet drop vs. range for various ballistic coefficients and muzzle velocity at 3,000 ft./sec; Fig. 5 shows bullet drop vs. range for various ballistic coefficients and muzzle velocity at 3,500 ft./sec; Fig. 6 shows bullet drop vs. range for various ballistic coefficients and muzzle velocity at 4,000 ft./sec; Fig. 7 shows bullet drop in inches at 100 meters vs. ballistic coefficient at various muzzle velocities; Fig. 8 shows ballistic coefficient vs. muzzle velocity to match drop of reference bullet at 100 and 150 meters. Description of the Preferred Embodiment Figures 1 and 2 show the invention embodied in a practice cartridge for 7.62 mm machine gun ammunition. Fins 11, 12, and 13 extend along the length of the bullet forward of the center of gravity. The center of gravity is in the cylindrical base 14. The front of the bullet is tapered toward a point 15. The fins extend forwardly of the base 14 and radiate outwardly from the tapered front of the bullet. The embodiment of Figures 1 and 1A has three equally spaced fins. The embodiment shown in Figures 2 and 2A have four equally spaced fins. The criteria for design of an exemplary practice round emulating the characteristics of the standard 7.62 mm bullet (150 grains) are discussed below. The requirements are that the bullet be stable and accurate out to the 50 - 150 meter range. Upon reaching this range the bullet should become unstable and tumble rapidly to the ground. The contributing components for dynamic stability of a spinning projectile are separated into the following categories. The stability factor is given as: 2(C L + k a ~ 2 CM p ) s d -2 C L -C D -k fc (CM + CM ) where S d = stability factor o C = lift coefficient L k = axial radius of gyration a CM = magnus moment P C = drag coefficent D k = transverse radius of gyration -5- CM = moment coefficent CM = static moment coefficient The contributing components are spin rate, destabilizing moment, and cross coupling of projectile inertias. Spin rate reduction requires lateral surface interaction of bullet surfaces during the time of flight of interest. The destabilizing moment is caused by the magnitude of the aerodynamic force and its displacement in front of the center of gravity. The bullet inertial properties are difficult to change in flight without costly moving parts. In accordance with the invention, the bullet (projectile) travels in two modes. In the first mode, the projectile will be spin stable and of sufficient drag such that it will fly a matching trajectory for 50 to 150 meters. At some point in this range, it will change abruptly to the second mode. In the second mode, the projectile will be characterized by a sharp increase in drag centered at its nose, which in turn, will significantly increase the destabilizing moment causing tumbling and curtailing projectile flight. The following ballistic parameters were determined for the exemplary round: range (meters), remaining velocity (ft./sec), remaining energy (ft.-lbs, total drop (inches), elevation in aim to strike target at given range (minutes of angle), maximum trajectory height above sight line (inches), time of flight, and ballistic coefficient. First assumptions included the following inputs shown in Table 1. TABLE 1 BALLISTIC MUZZLE VELCOCITY COEFFICIENT FT./SEC. C 4000 .50 3500 *.409 3000 .20 *2750 .15 2500 .10 .05 Figures 3 through 6 depict trajectories shown as bullet drop in inches from horizontal vs. range in meters up to a max of 150 meters for ballistic coefficients ranging from .05 to .50. These curves illustrate the effect of ballistic coefficient and muzzle velocity variations on bullet trajectories compared to the reference ballistic coefficient of C= .409 at a muzzle velocity of 2750 ft./sec. The effect of ballistic coefficient (C) and muzzle velocity on trajectory, even at this relatively short range of 150 meters, is striking. At a muzzle velocity of 4000 ft./sec, the total difference in drop between C = .50 and C = .05 is approximately 2 inches. The difference increases to approximately 5.8 inches when velocity drops to 2500 ft./sec. These effects become minimal at the 50 meter range and somewhat greater at 100 meters, particularly at the lower velocities. In essence, the curves shown in Figures 3 to 6 display a parametric envelope of trajectories, ballistic coefficents and muzzle velocity variations wherein the reference trajectory serves as both the upper and lower limit. For each muzzle velocity there is a ballistic coefficient that most nearly matches the trajectory of the reference bullet. The reference bullet trajectory is based on the characteristics of a .308 diameter, 150 grain Spitzer bullet with a ballistic coefficient of .409. This bullet is considered to have trajectory characteristics close to the military 7.62 standard. Three additional useful curves were developed from the data. The first. Figure 7, shows bullet drop at 100 meters as a function of ballistic coefficient at various muzzle velocities. A ballistic coefficient to produce the same bullet drop as the reference at each muzzle velocity was determined by noting the crossing point of each bullet drop vs. ballistic coefficient curve with the reference bullet drop line. Figure 8 represents the same data as Figure 7, except the bullet drop for the reference bullet was taken at 150 meters. Figure 9 is a distillation of the information shown in Figure 7 and 8, except ballistic coefficient is shown as a function of muzzle velocity to match drop of the reference bullet at both 100 and 150 meters range. The zone between the cucves represents ballistic coefficient values to be considered at a particular muzzle velocity. It is evident from Figure 9 that trajectory matching of the practice ammunition bullet to the standard 7.62 mm bullet will be more sensitive to ballistic variability in the muzzle velocity range of 2750 to 3000 ft./sec. than it will be above 3000 ft./sec. muzzle velocity. The foregoing analysis was made to serve as a starting point, but the ballistic coefficients must be approximated experimentally. While a particular embodiment of the invention has been shown and described, various modifications are within the true spirit and scope of the invention. The appended claims are, therefore, intended to cover all such modifications.

Claims: Cla ims : 1. An explosive round for a rifled gun comprising: a cartridge containing an explosive, and a bullet for said cartridge having a plurality of fins extending along the length of said bullet forward of the center of gravity thereof. 2. The explosive round recited in claim 1 wherein said fins decay the spin of said bullet imparted by the rifling of said gun. 3. The explosive round recited in claim 1 wherein said bullet has a cylindrical base with the front of said bullet being tapered toward a point, said fins extending forwardly of said base and radially outward from the tapered front of said bullet. 4. The explosive round recited in claim 1 wherein said bullet has three equally spaced fins. 5. The explosive round recited in claim 1 wherein said bullet has four equally spaced fins.

Inventors: LESHNER ERVIN, LESHNER, ERVIN
Applicants: ACTION MFG CO, ACTION MANUFACTURING COMPANY
Application year: 1986
Main classification: F42B11
IPC codes: F42B10