Heat Shield Technology
Would the drop in BC you claim to see be caused by the normal change in BC at lower velocities?
No, it would not. The drop in ballistic coefficient (BC) associated with a bullet flying down range as it slows down is driven by two things. First, it is caused mostly by the drag coefficient (Cd) versus Mach number (drag model) of the standard not being at all like the actual drag coefficient versus Mach number of the projectile that is being evaluated. In the case of G1, at Mach numbers above 2.0, the slope of the Cd versus Mach is fairly close to modern boattail type projectiles and gives reasonable approximations. Below Mach 2.0, the G1 Cd vs. Mach becomes more and more dissimilar to a modern boattail type projectile, and this is why the BC numbers drop. The problem is not as bad with the G7 drag model. Unless the projectile being evaluated matches the shape and Cd vs. Mach of the standard projectile being used, errors are going to occur. The phenomenon we were seeing was happening immediately and was happening rapidly. It was totally inconsistent with the known problems with using BC.
Secondly, another problem arises when testing is done to determine multiple BC values. Testers usually take a standard twist rate for a given caliber and download to shoot at lower velocities. This results in abnormally slow spin rates on the projectile being tested for the velocity it is traveling, which changes the gyroscopic stability, muzzle tip off, body shank wear and limit cycle yaw, further muddying the whole BC issue. The spin of a projectile typically drops off at a dramatically slower rate than its velocity, causing an increase in gyroscopic stability (Sg). For example, using the PRODAS 6 degree of freedom (6 DOF) trajectory analysis on the 6.5 mm 140 ELD™ Match projectile, discussed below, fired from a 1:8” twist barrel at 2,780 feet per second (fps), has a spin rate of 26,272 radians per second (rad/sec) at the muzzle. At 800 yards, the projectile still has a spin rate of 22,145 rad/sec, nearly 85% of the muzzle value. Yet the retained velocity is only 1,825 fps, 65% of the muzzle value. If the same bullet were fired from the same twist barrel at 1,825 fps, the spin rate would be 17,214 rad/sec, 22% less than it should be at that velocity. This also makes the stability of the bullet 22% less than it should be at that point in its trajectory.
We never used G1 from the beginning because of its gross differences in drag characteristics to the aerodynamically efficient projectiles we were testing. This problem with BCs is why you will see us discussing projectile performance from now on in terms of the drag coefficient. It is the radar-generated exact total drag for the projectile being evaluated.