Colin the last time I measured my BC with jsb 8.4's using 2 chrono's I got .0267. but that was a while ago and I sure I am getting a better one now? I will do a retest as soon as I get chance.
Aiming using the cross hair at every distance from 8 to 45 then measure in mm the poi from the cross hair aim point. Then to plot that trajectory in CG I have to increase the BC in CG even more than that to get the trajectory to go through all those intersects but the trajectory is also influenced by the twist rate and gyroscopic stability not just the velocity and BC.
But gyroscopic stability can work against you, the pellet has to follow the trajectory nose first, if it starts to drop and is still flying horizontal ie the pellets axis is not in line with the pellets path then the skirt becomes visible to the frontal area and the rate of deceleration increases massively, also an unstable pellet will slow quicker for the same reason. I experienced this with die 22 in my rifle, very accurate and the same aim points upto 40 yards then a huge drop between 40 and 45 yards, But find the right combination of barrel pellet and velocity and there are benefits!
You cant eliminate the effects of wind or gravity, only reduce them!
If you said 2 rifles firing the same pellet at the same velocity will have the same external ballistics, I could agree with you but then we would both be wrong!