I will beg to differ. Neither muzzle velocity nor BC are fixed known values, despite what chronograph manufactures or bullet manufactures might try to tell you. Most all chronographs are +/- 1% of actual, presuming a perfect setup, which most users fail to create. BC is somewhat variable, depending on start velocity, twist rate and yaw. Muzzle breaks and suppressors can alter BC, the mfg gives you a number based on a test barrel at a (perhaps) stated velocity. That's not sure to be the number from your rifle.
For example, we just finished an XLR class here at Gunsite. After class, I fired up Genesis and launched a few Warner Tool 351gr solids from my .375 CT. I use a Labradar, carefully positioned, the scope is calibrated on a fixture, the target ranges accurate to +/- 2 yards, WT provides a BC derived from Dopper radar data. The elevation at 2307 yards is wrong by .4 mil, 1501 meters is off by .3 mil, using the Labradar velocity and the factory BC.
What's wrong? The model is not faulty, I've used Coldbore for many years, it's one of the recommended software systems here. All of the basic systems available all have good and bad points, but most provide good solutions if correctly calibrated.
My FFS profile is correct for the .375, but it required an alteration to the 'factory' BC and a change to the DK value. Doing nearly the same changes to the Genesis program corrected the solution. I've got to use it more, but I don't really expect to see any problems with the engine itself.
Now, I should note that changing the MV by more than a couple percentage points is suspect, as is altering BC more than a couple percent without solid downrange velocities/TOF to compute corrected values. Changing MV from 2560 to 2500 to correct a .3 mil error at 600 is clearly wrong and is going to cause more problems than it solves. Changing the MV by 10 fps to move .2 mil at 1500 yards is quite reasonable, and hardly indicative of a faulty model.
Try for example, putting three LabRadars in position on one gun (L/R and above) Do they all read the exact same value? Which one is correct? Even the 8' spacing on an Oehler is subject to setup errors in the angle of the sensors.
Let me start by saying that the term "truing" is correct if applied correctly while "tweaking" is not when using some sort of "software adjusted MV" a really bad practice that tracks down to the now long gone Exbal, which is still prevalent in the realm of Point Mass solvers, clearly a practice that has nothing to do with the correct technical approach by relying on a methodology so common in the field of engineering when dealing with instrumental errors.
I certainly agree that using the DC value is the right way to "true" a somewhat dubious BC. Having said that, MV is another story.
Yes, of course, MV is as good as the instrument and the data analysis we use after collecting a statistically significant sample (in my case, no less than 20 shots coming off my best accuracy developed load) followed by a good dose of number crunching. I've been using this technical approach for many years and never had any trouble with great predictions up to 2 miles (not in a match and during almost zero wind condition)
Data Uncertainty is the demon we fight every shooting session, but there are methods and best practices to deal with it meant to obtain the best possible fit. Instrumental built-in error is something we can do nothing about and the only way to follow is to apply what we learn from the science of statistics. My personal keywords are sample size and lots of samples as possible.
I agree with what you said about adjusting MV, I wouldn't say it better, but I guess that you are also against any computer-guessed value when it comes to MV, so common to see these days. That's is the whole point of my previous post. What about temp swing affecting MV? What about many other error sources, as mentioned before, are the LR shooter not evaluating correctly? The farther the target the greatest the error, as we all know. In short a tiny adjustment is OK as long as it based on some technical criteria.
Another thing I noted very often is how shooters deal with absolute values vs the inherent and non-absolute nature of shooting. I mean, is really a 0.4 mil difference an error or it's not? To answer this question to myself I go back to the Error Budget module of CB and play with in order to have a real understanding of the expected group size under the uncertainties of my inputs which are going to be there I like them or not.
In other words if the prediction is inside the CEP or expected group size then it's a good one, otherwise it's not. Groups will only grow up as distance increases and only because of the said uncertainties, independently of how "accurate" the rifle or load are.
We certainly know that fact, but most of the time we fail to establish a baseline to compare with and say how good are XXX software predictions vs YYY and that's an exercise I do as often as possible and why I run CB and now Genesis, because it's the most solid predictor I've ever used so far (including FFS) and way better than any Point Mass solver, even when loaded with Doppler data. Not saying Point Mass solvers aren't good, they are but have lots of quirks and one quite notable is the need to resort to the infamous "truing" a practice that makes shooters to perform a voodo-like procedure.