Ya, I was taught the manual version of truing custom drag curves before they were programmed into AB in the 5700 elites. We just did it manually and had stepped BCs for different velocities. Understanding the transonic window, prior to subsonic, is a function of CDMs. If you use CDMs in AB but also disagree with what AB says about transonic windows you're kinda being a dumbass. I'm not going to hitch my wagon to AB and say that everything their teaching is irrefutable fact. I've been taught all kinds of shit that ended up being proven untrue over the last 22 years doing this professionally. The old high angle formula, the old thought effect of spindrift that ended being proven to be a Leupold M3A and M3LR mechanical effect, and more. I'm still waiting for someone to disprove AJ. I just turn winds off and don't even bother factoring into data. But maybe I'm just being lazy. That's why I asked what MK20's source was. I'm open to learning despite whatever expert status and experience I like to think I have. If he comes back with data driven explanation from a reputable source then I'd be interested. If it's just I shoot a lot and have been around a while then I'll chalk it up to an old guy with an online presence to defend. There's a lot of us who shoot a lot and have been around for a while. That's not going to be convincing.
I couldn't agree more. But I'm also not experienced enough to know how to build a Custom Drag Model besides sitting there and shooting rounds at a target within the transonic window. So for those who don't know -- and myself now researching how best to build CDMs -- I'm leaving the info below here. So it sparks an interesting debate: use a round that is supersonic at 1000 yards but doesn't group as tight, or use a round that groups much tighter, but needs a CDM to hit 1000 yards on target consistently?
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Custom Drag Models (CDMs) are used to model the flight path of bullets, and are particularly important for accurately predicting trajectories beyond the transonic range. The transonic range is when a bullet slows below Mach 1.2, or around 1340 feet per second (fps).
Here's some more information about CDMs and the transonic range:
CDM accuracy
CDMs can provide highly accurate predictions, with one example showing a match within +/- 9 inches for a bullet traveling up to 1323 yards.
CDM generation
CDMs are generated using Doppler Radar to directly measure a bullet's drag profile.
CDM availability
CDMs are available for all bullets in the Applied Ballistics software ecosystem.
CDM benefits
CDMs eliminate the variation between a bullet's actual drag and the standard model being used.
Transonic range challenges
The transonic range is difficult to predict because of the mismatch between the standard G1/G7 drag curves and the bullet's actual CDM.
Transonic range effects
The transonic range can cause bullets to yaw and destabilize. Short, flat base bullets are less affected by the transonic range than
long, thin bullets.