Here are a couple of fun facts while you think about this:
1) A BC is velocity dependent. Manufacturers can cherry pick this (Some don't like Berger, Lapua, or Sierra) We also do the testing for a number of companies like Berger & Lapua and many others, but picking the right BC can be tough. While our high resolution radar tracks a bullets velocity every couple of feet which can literally provide thousands of data points, check out this abbreviated calculation and look at how the BC changes throughout the flight.
Also, while you look at the data, notice the blip at 949fps. Our data is usually the average of 10 - 15 shots, so this isn't a single bullet anomaly. The transition from Super to Sub occurs over a period from Mach 1.2 to 0.9. Its not just a solid "wall" that you slam through.
By looking at this, which BC would you choose? Usually a shooter wants one that is most representative of the distances they will be shooting. You can also see how it leveled off in the subsonic.
The important take away here, is that a BC is actually Mach dependent (Velocity & Speed Of Sound(Temp Dependent)) and that it changes a lot during a bullets flight. In this case we have a high of 0.382 and a low of 0.332 G7. For a G7 Bullet. Take a look here and you will see how much the BC varies from the standard models for this bullet throughout its flight:
This image shows you how consistent the difference is between the bullet and the G7 standard in supersonic, and how consistent the difference is in subsonic. Indicated by how relatively flat the lines are. Relatively being the key word as you can still see the bumps and dips in the variation line. At transonic you can see its varies a lot during that short time through the transition. If the bullet was a perfect fit for a G7 model, that line would be perfectly flat.
Essentially, when you pick a single G# and input it, you are telling the solver that line is "flat" with no variation in that BC over time from the standard. Which just isn't the case. The band-aid for this is stepped BCs. Picking 3, 4, or 5 BCs and inputting that instead. To make up for the non linear difference. But why use a handful and still miss out on the rest of the data, when a solver can actually just use the flight model directly off the radar and get dozens upon dozens of data points. Including high resolution modeling during the transonic shift. (Which is what we do).
The G1 line is just a mess, and you can see why a boat tail bullet should be using a G7 and not a G1 BC when using a BC.
This is why we advocate for the use of a CDM and to only use an Averaged G7 BC for supersonic calculations or when trying to compare two bullets. This bullet is pretty good for consistent variation, and some have far more variation. It isn't a bad thing, it just means it doesn't follow the G7 bullets drag model perfectly.
Hopefully this helps to show you how a BC is velocity dependent, and changes through most of the bullets flight with the biggest variation being in the transition from supersonic to subsonic. Also it hopefully shows why picking the correct form factor (G1 vs G7) is important.
