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Shoting at a fixed distance & calculating back the trajectory just gives you a different view of the computer model, again, not real world.
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Yes and no. Unlike just using the computer model, having knowns removes some of the problems of solely using the computer program. Turns out, claims of manufacturers are not always 100% and so when you plug in their data, the results from the program can be of significantly from reality.
So you shoot a couple of known distances. If the results of the program along the trajectory do not match the reality of the two results actually obtained, tpcollins can tweak the parameters of the program to match the two known impacts and then the trajectory provided based on the two actual knowns should be very close to what will be the real world result, which sounds like it should be more than sufficiently accurate for the caliber being used within the short shot ranges. That information can be correlated with the reticle which can be field checked for its accuracy by noting distances between the lines at a fixed distance or distances.
I am all for shooting a target every 10 or 20 yards which I have done for 5.56 out to 200 yards. It is enlightening, time consuming, and in the end, the refinement wasn't amazing. The quarter and half inch differences really didn't turn out to be of any significance.
So it isn't absolutely real world every shot. You are correct. However, with two knowns, you can work out a realistic trajectory with the ballistics calculator that will likely be more accurate than the shotgun ammo, depending on the ammo and the rifle from which it is fired. The method works best with the known distances being longer, so for tpcollins, he might want to do 100 and 150 yards, compare with the trajectory program and everything from 150 and back should be very good and from 150-200 should be functional as well.
tpcollins, I have shot with 4 or 5 BDC scopes. Two I have shot with the correct ammo and rifles (resulting in the correct velocity and related variables). In the cases where everything matched, the BDC was reasonable, but far from perfect and certainly the scopes didn't change with the shooting conditions. The hotter the day got, the further off the scope was. That will hapen with any fixed BDC or normal reticle. The POI will change with changing conditions.
You change the ammo, gun, mounting height of the scope and things can be off significantly. The lines are just that, lines, when you don't have everything that is right (original parameters for the origical setup of the BDC), but that does not mean they aren't just as useful. You just have to recall that the 100 yard distance in the reticle is really accurate for 88, 200 is for 190, etc. (whatever your results are).
Here is a BDC reticle from a Trijicon ACOG with my POI at various distances using this with a Remington 788 firing Lithuanian .308 surplus ammo. I did verify every dot for the distance indicated. 200 yards was the actual maximum intended used distance and chances were shots were going to be inside of 100 yards. So my setup had everything happening withn 5 MOA on my scope, but I could put the top of the reticle on the same place of the animal (hog) out to 150 yards and get the desired result. The point here is that depending on the use, the computer statistics, if correlated with a couple of knowns, aren't apt to be off enough distance to matter for the shotgun situation. His POI spread will undoubtedly be somewhat larger, but to get the two knowns to match with the trajectory program, the trajectory track is going to have to be awfully close between reality and what is calculated.
What is interesting is that the donut (in red) is supposed to be 4 MOA. Verification of your BDC on the target with a ruler can be enlightening. It is closer to 5. The scope is actually for 62gr. M855 ammo fired out of a shorter barrel. However, on the previous gun using a 24" barrel, I was shooting Privi Partisan 69 gr. ammo and the POI dots shown here were all very close to 200 yards. I tweaked my cheatsheet image slightly for the new gun and ammo, but not much.
All this was even more confusing when realizing that the measurements of the BDC in the ACOG were problematic. The manual refers to distances in both yards and meters, introducing a possible error of about 9.4%. You can read Trijicon descriptions today about distances in meters, but give fields of view at ranges such as 100 yards. Adjustments are X clicks per inch at 100 yards, but the reticle is apparently in meters. How is that for wonderful?
http://www.uscav.com/pdfs/acog.pdf
I shot a Springfield scope, an older model, that had its reticle lines at every 2 MOA as I recall and they were actually closer to 2.7 MOA when put to the ruler at the target.
So if you don't want to shoot a lot because you have really expensive ammo, I would shoot 3 shots at 2 known distances and record the difference between the POA and the average POI of the group at each distance. Ruler verify that your reticle is correct as per claims or any measurement about which you will based making a shot. Plug in the chrono data and the BC for the ammo into your ballistic calculator along with all the other relevant data necessary and see if the calculator predicted accurately the impacts at the 2 known distances. If not, work backwards from the distances to determine your trajectory. As your BDC likely is not set up for your setup, you can make a numeric card cheatsheet to go with your setup or do like I do and make a picture version. It requires less cognition in the field to figure out where to aim for your actual sight picture, but the numeric method is the traditional method used by most hunters.