Why Use Multi BC's
Why Use Multi BC's when modelling trajectories of highly efficient Lead Free Bullets.
I have been asked on many occasions why I use a multi bc solution for my TLR bullets for modelling trajectories and wind deflection, so thought it might be interesting (to some) to write a short note to illustrate the reason, including a real world examples. As many will already know the G1 BC of a bullet, particularly a highly efficient lead free bullet varies considerably with its velocity. So how does this affect trajectories and wind deflection in the real world? By using a multi BC solution you are essentially splitting a trajectory into much smaller sections and then 'stitching' them together. The upshot of this methodology is that the error in the predicted performance of the bullet (over the defined testing range) is so small that it is indistiguishable from the real world trajectory. A simple initial example would be as follows: My testing for the 6.5mm 114 TLR that I produce was done using a 6.5x47 rifle producing a muzzle velocity of 2,920 fps and a 6.5 PRC with a muzzle velocity of 3,240 fps. The multi bc solution resolved from that is as shown below:
If you were to test the 6.5x47 in isolation at 500 yards (distance at which the bullet has slowed to 2,000 fps from the 6.5x47) looking for a single G1 BC you would confirm a G1 BC of 0.49. (Essentially you measure your MV and your impact velocity and extrapolate the result.) When compared to the multi BC solution at the test muzzle velocity of 2,920 fps the results from the single BC are pretty close. Wind speed is 10mph and direction 90 degrees in this example. Spindrift has been ignored as its magnitude would be the same for either example.
The problem arises when you try to apply that single BC to the bullet when it is fired at a significantly different muzzle velocity. In the real world example a 6.5 PRC with an MV of 3,240fps. When you do this, the differences between the multi BC solution and the single BC become significant. In this case the table goes to 700 yards because that is the point at which the velocity gets down to 2,000 fps.
As is now evident the single BC elevation and wind deflection are significantly different to the multi BC solution. In real world absolute terms approximately 11 inches in elevation and 9 inches in windage. Real world testing of the multi BC solution through many rifles at many different MV's has proved it to be a very accurate reflection of trajectory and wind deflection.
As a bullet manufacturer I have to provide a ballistic solution that works at all sensible/practical muzzle velocities for a given calibre and this is the only way to do it in my view.
If you are shooting a lead free (efficient) bullet at the same or similar MV to that at which it was tested to produce the published single BC the predicted trajectory will be close to the actual trajectory and useable out to significant practical hunting ranges. If you are not and there is a significant difference between your MV and the test MV the error in a single BC solution becomes significant.