G
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Well I guess we need to look at a REAL authority on ballistics but first:
You will see in print and on line that BC is the most important factor in computing wind drift. THAT is generally a true statement but it is not true that BC directly relates to accuracy or that BC "determines" wind drift. This is because BC determines TIME OF FLIGHT and everything else is relative to TOF.
From here:
USN Exterior Ballistics
This:
The equations require (in addition to values of initial velocity and angle of departure) a value known as the Ballistic Coefficient, C, for their
solution. This coefficient is a measure of comparison between the retardation of the specific projectile for which the range table is being prepared
and the retardation of a projectile of a specific standard form in air of an arbitrarily chosen standard density. The expression for C is:
C=w/id(squared)
where
w = weight of projectile in pounds.
d = diameter of the projectile in inches.
i = coefficient of form (the ratio of the retardation of the given projectile to that of a projectile of standard characteristics).
The retardation of the projectile of standard characteristics which is being used as a basis for comparison is available in either the form of a drag coefficient or the form of a resistance curve. The data for such a drag coefficient or resistance curve are available from measurements obtained by actual experimental firings of the standard projectile. It should be noted that i will include not only retardation relations based on form, but any factors, other than weight and diameter, which affect retardation.
I have highlighted the parts to which you need to pay particular attention.
Let's talk about what this implies because what it says is patently obvious and has ABSOLUTELY NOTHING TO DO WITH ACCURACY, NOTHING AT ALL, PERIOD.
The BC is a number which is paired with a drag model that relates the projectile under study to a reference projectile. It EXPRESSLY relates the "retardation" of the test projectile relative to the "retardation" of the standard projectile used in the drag calculation.
Because it relates the "retardation" of the test projectile to the standard projectile WHEN it is used to calculate said "retardation" what drops out of the equation is an estimate of the change in velocity (Delta V) of the projectile under test at some time (T) after launch. This is COMMONLY referred to as "time of flight" (TOF). After a certain TOF the remaining velocity of the projectile can be projected/predicted RELATIVE TO THE STANDARD PROJECTILE.
A ballistics table such as the one Chairgun or Strelok provide is an iterative calculation. You have to do it that way because computers can not do calculus (for reasons which, while interesting are not relevant here). With a computer you actually have to program the math. So the algorithm calculates the velocity of the projectile starting with the muzzle velocity and moving forward from the barrel at time T, T1, T2, T3 ... Tn. These numbers are actually based upon the distance the projectile travels with the starting velocity. IN OTHER WORDS, they are an APPROXIMATION. The programmer assumes that the projectile has traveled some distance, lets say one foot for our purposes, at the muzzle velocity and then computes the TIME THAT TOOK (TOF). Then using TOF he calculstes the drop due to gravity in that time. That becomes the APPROXIMATION for the velocity at one foot from the muzzle. In this particular example the RESOLUTION of the model is ONE FOOT. Another programmer might choose one inch and do his calculations for each inch of travel.
Now then there is a curve (an equation for a curve) which describes the "standard projectile" (the model G1, GA ...). That curve defines the drag model for that standard projectile. The "BC" relates the drag of the test projectile to the drag model in use. So once the programmer calculates the velocity of the standard projectile all he has to do is multiply that product by the BC to get the new velocity of the test projectile at the same TIME OF FLIGHT (not the same distance, the same TOF). Everything in these equations is time based because TIME is the only constant in the equations, everything else changes.
Now that we all understand that everything is time based in these equations you can see the relationship between BC and time of flight. Remaining velocity is first computed for the standard projectile at some distance (d) and then time of flight to that distance is calculated and then distance traveled is computed for the test projectile and then remaining velocity is calculated for the test projectile ... rinse and repeat for each distance ... That's how the equations work and that is exactly how they iterate as the model travels down range.
Lots of things fall out of this. First we see that EVERYTHING is a function of TOF. Everything depends upon TOF EVERYTHING. Drift, drop, remaining velocity, distance traveled, EVERYTHING is a function of TOF. Second we see that BC clearly relates to one thing only, the standard projectile. The ONLY thing BC does is modify the distance traveled in the TOF equation. That's is it. PERIOD. If the distance traveled at some velocity (V) for the standard projectile is some number (N) then the distance traveled by the test projectile in the same TOF is BC times N AND THIS IS AN APPROXIMATION.
What does BC NOT tell us? BC tells us absolutely NOTHING about the wind drift characteristics of the projectile except in head and tail winds where drift is actually zero and wind adds or subtracts from drop.
Let's talk about WIND drift for a moment. Wind drift is principally a function of TWO things. The first thing is the side projection of the projectile. Consider two pellets having the same BC, weight and same caliber, shot at the same velocity, from the same gun, one having a tiny wasp waist and the other with a more conical section like a "monster". The one with the "wasp" waist will very likely experience LESS wind drift BECAUSE that side profile presents LESS area for the wind to act upon.
How do we calculate wind drift? Well you see THERE IS NO SIMILAR CONSTANT FOR BC in the equations for wind drift. It is a straight acceleration calculation using TOF and the drift measured for the standard projectile. That is to say, in my couple of decades of doing such calculations, I personally have never run across any model which contained a constant like BC used to compute wind drift relative to the standard projectile (the model G1, GA ... ect). Maybe such a thing exists and someone will be kind enough to point me to it...
The equations we are talking about here are not that complicated or hard to understand but there are multiple equations all working together to produce the information we want. There are a number of interactions and there are different ways of LOOKING at the math. The one thing which is always CONSTANT in these equations is the passage of time and therefore THAT is what scientists and programmers consider the base premise. It is all about TOF.
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