Mike,
"A difference of 10 inches at 1200 yards has been cited as exemplary
difference between the two fairly well recognized calculation sets."  this was only example and using only a more common approach of calculating trajectory based on BC and retardation coefficient n of 0.5. Under realworld conditions Mayevski tables would be off by far more, compared to what you can get with Pejsa approach. Obviously at least in theory, it needs verification in practice (someone must have done it by now). Oh, and if you're saying Pejsa is wellrecognized calculation set, then I guess there should be less resistance to accepting his approach :)
When it comes to atmospheric pressure I think most people fail to realize that it's not the pressure that affects the bullet in flight, it's the density of air. It is said that standard atmospheric pressure at sealevel is 29.92 inches. But everyone knows that during the storm or unusually hot day atmospheric pressure would change. The "standard" atmospheric pressure is merely a reference point. A combination of humidity, dew point, temperature and atmospheric pressure would affect the air density, which in turn affects the bullet in flight.
If you use Pejsa program you can enter your altitude there (or just the pressure).
It seems to me that the best approach to account for air density is not to use straight atmospheric pressure (or altitude) readings. Instead, it's much more effective to use density altitude (see this for some references http://www.nkhome.com/knowledgecenter/viewtopic.php?t=225). Then, assuming you're not shooting at an angle and you're not shooting into a heavy fog that lies 500 yards away, the trajectory calculated will be quite accurate. If the air density changes substantially from your shooting location to the target area the trajectory may differ slightly, but most likely much less than if you were to use straight atmospheric pressure readings. Interestingly enough, Pejsa says that atmospheric pressure itself has far less effect on bullet trajectory than rotation of the Earth. Currently his program doesn't account for it. He mentioned that in next year he will be releasing a new book that will have additional information on the subject. Not sure I want to buy the book just to find those details out, I would hope that new version of software would instead account for this (if needed).
"If I have already shot my data, why would I need to risk a downrange
chronograph when I already know that JBM, ATRAG and Sierra are within
the circular area of probability that is my target, using standard drag
function models.?"
If you're saying that you already know exactly how your bullet flies, then you already got your answer  you don't need chronograph, Pejsa software or even BCs of your bullets. If you can hit the target anywhere within the effective range and you're satisfied with accuracy, then certainly there is no need for anything else. Furthermore, if you're rifle doesn't have subMOA accuracy, you may not even see the difference at 1000 yards.
However, if you reload quite a bit and experiment with different powders, bullets and so forth  it gets complicated very quickly. Here is a simple example. Using JBM pick some standard variables and calculate trajectory of a bullet using G1 and only BC of some known Sierra bullet. Then do the same, but this time pick the bullet from drop down list. See the difference in results.
For example, say take the Sierra MatchKing 175gr, 30 caliber bullet, use BC of 0.505, muzzle velocity of 2850fps.
First do a calculation with G1 and use only the BC, don't select the bullet from drop down list. Then run second calculation, this time use the dropdown selection.
You will see that for BC 0.505 at 1000 yards it calculates the drop of 330.3 inches. Same thing when the bullet is selected from drop down list shows 334.9 inches
What happened here? Well, most likely the calculation from the database uses three difference BCs and adjusts the calculation as the bullet speed drops by using different BC. And therein lies the rub. What if I'm shooting Lapua 11 gram 30 caliber FMJBT bullet? I only have one reference BC and that means that even if I presume Mayevski table to be valid, I should expect an error of say 4 inches. If my rifle can only maintain 1 MOA accuracy and I'm shooting out to 1000 yards, it means that I may be potentially 14 inches off the target. If my target is 12 inches, that means I will most likely miss and get frustrated trying to hit it. And what if I shoot a bullet for which I have no known BC at all?
So, if Pejsa helps create a more accurate trajectory table for your particular load, your scope reticle doesn't shift during adjustment and UFO with Kucinich on the way to campaigning in Iowa doesn't fly over your shooting range leaving pocket of ultradense air in its wake  your chances of hitting the target increase substantially.
Edited by spreader
