Keep in mind I am not a good shot and have very little experience with load development or competition but I have done OCW and watched many videos and have seen many forum posts of OCW. In the tests I have seen, over a charge weight range the POI will rise and fall, so positive compensation occurs on the down slope of charge vs POI @ 100 yards? Can't you calculate the distance at which positive compensation would be most effective if you just used the difference in MV and POI of 2 points on that down slope?
For instance:
if my 223 (80 vld, 2950 fps) has 6 MOA of elevation needed at 400 yards
And if my 223 (80 vld, 2850 fps) has 6.75 MOA of elevation needed at 400 yards
So for the best vertical at 400 yards, I would want my 2850 charge to impact .75" higher then my 2950 charge at 100 yards. More than .75" means my optimal positive compensation occurs at > 400 and if it is less than .75" then my optimal positive compensation occurs at < 400.
And if my 2950 impacts higher then my 2850 charge then I just don't even try using it at distance since it will be worse (negative compensation?).
So in theory the more rigid the system, the smaller the slope will be, which is good for tactical shooters who shoot at different distances but bad for F Class / BR guys who shoot at 1 distance?
Or am I completely misunderstanding this?
This is not a direct answer to your questions, which I believe could only be done reliably by actual testing, without having a lot more information. Mechanistically testing the effect of positive compensation is beyond the scope and equipment available to most shooters. As an example, barrel harmonics are affected by barrel stiffness, which is dictated by contour and caliber. So predicting (or measuring) muzzle launch angle for a given load in a given rifle depends largely on how that specific rifle is setup. It's not something that just everyone can go out and measure accurately.
What you can do relatively easily is test empirically, and let the targets tell what works and what does not. That is why most of us are restricted to using various reloading approaches and shooting targets to decide what the optimal loads are. I think the most important you might begin to develop a feel for is about how wide a positive compensation load window is in terms of velocity spread. In other words, within a charge weight window that clearly show positive compensation on the target, what is the velocity spread? How far apart can two load be in velocity, and still produce impacts at approximately the same height on the target. Certainly there must be an upper effective limit over which that process is effective. Those are not numbers that you see mentioned very often, but might be very useful over the long haul.
As far as your questions about PC at various distances, there are many BR shooters that insist on doing load development only at the specific distance at which they shoot. Their comments suggest that a load that shows good compensation at one distance may not be so good at a different distance. The general trajectory of a rifle bullet under the influence of gravity is a parabola. Positive compensation allows for a slightly slower bullet that exits the bore with a slightly higher muzzle launch angle to hit the same vertical spot on a target at some distance as a slightly faster bullet that leaves the muzzle a fraction of a second earlier when the muzzle launch angle is slightly lower. The first bullet is slower, but it's launched with a higher arc. The second bullet is faster, but it's launched with a slightly lower arc, so they arrive at the target at approximately the same elevation. As with the maximum velocity differential over which this effectively occurs I mentioned above, I can also imagine that this effect is dependent on launch angle. In other words, charge weight and their resultant velocities (i.e. barrel occupancy times) and their coincident launch angles that exhibit a very good positive compensation effect at 500 yd may not be the exact same ones that provide the best result at 1000 yd. At least, this is what I surmise from the comments of those have observed that optimal grouping tends to be distance-specific. If the effect was identical at two different distances, it could be argued that a ladder test fired at 1000 yd would be expected to look almost identical to one fired at 500 yd, except with the impacts spaced farther apart. However, we know from actual testing that that this is not always the case.
Unfortunately, not everyone has the ability to test at the farthest distances they will shoot, whether that may be due to range access, wind conditions, or access to appropriate targets. So many of us have no choice but to test at shorter range and hope the results will translate over wide range of shooting distances. Whether that is true probably depends in a large part on the precision requirements for the specific discipline you're shooting. Nonetheless, the effect of barrel harmonics is very complex. In such a case, it is sometimes easier to simply do standardized load development and empirically determine what works best, rather than attempt to define the exact mechanisms of "how" and "why" to the nth degree. I would love to have access to the equipment and facilities to carry out far more involved and extensive testing than I currently do. However, for my sanity, I have reluctantly accepted the fact that it is unlikely I will be able to do so anytime in the near future
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