wncchester claims:
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The velocities in factory ballistics charts and our loading manuals are often so far off reality that any efforts to 'calculate' a speed has nothing to start from.
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I disagree. The numbers the factory got are just as real as the numbers anybody else gets. If you don't understand why, then go measure a bunch of barrel's bore, groove and chamber dimensions. Next, go measure the velocity spreads several lots of the same powder produce with the same case, primer, powder charge weight in the same barrel. And finally, give the same rifle and ammo to several people and have them chronograph that load in that rifle. The issue of different neck tension's effect on muzzle velocity is a whole different matter.
You may well not believe the muzzle velocity spreads this simple and real test of components, barrels and people get.
We all choose different barrels, powder lots, primer lots and probably shooting positions than the factory did. Therefore, we should not expect to get the same muzzle velocity they do. We don't use the same "stuff" the factory did.
Plotting a graph is a good way to get an idea of what might happen.