Custom Drag Model(CDM) aka Mach vs (CD) vs G1/G7

[DocUSMCRetired]

Applied Ballistics has been providing custom drag models for use in trajectory prediction for the past several years. Custom Drag Models (CDM’s) for bullets are a more refined way of modeling drag for bullets because you’re not referencing a standard like G1 or G7, rather you’re using the actual measured drag of a specific bullet in a ballistic solver. This results in more accurate trajectory predictions especially thru transonic.

Although the benefits of this approach to modeling drag sound obvious, many questions have arisen as to how the custom drag models are different from BC’s, and what it all means. In this article, we start at first principles and build an understanding of how drag is modeled as a force, and how that’s applied in ballistic solvers to arrive at highly accurate ballistic predictions. Finally, the article concludes with some real world examples of how accurate ballistic trajectories can be in the real world when using CDM’s in the Applied Ballistics solver.

This article appears as Chapter 10 in Modern Advancements in Long Range Shooting – Vol 2. http://www.appliedballisticsllc.com/Articles/ABDOC130_CDM.pdf

 

[JPeelen]

Using a G1 BC means using the RED curve for trajectory computation. Below about 1800 fps (or Mach 1.6) in the above example the red curve predicts a much lower drag than the real drag. Consequently, any G1-based calculations will make you believe your bullet has faster speed, shorter time of flight and flatter trajectory than in reality.

 

[Bryan Litz Ballistics]

Using a G1 BC means using the RED curve for trajectory computation. Below about 1800 fps (or Mach 1.6) in the above example the red curve predicts a much lower drag than the real drag. Consequently, any G1-based calculations will make you believe your bullet has faster speed, shorter time of flight and flatter trajectory than in reality.

That's exactly correct, which is why you have to increment the G1 BC down to match reality at long range.