Having seriously looked at Ballistic Coefficient and any benefit it can produce for air gun use, it appears there is no constant BC for any same brand pellet. Minute differences in weight can alter the BC as can velocity so what is the point of persisting with its use. Consider that velocity variation is common even in PCP's fitted with a regulator, a springer with its wider variation has less chance of consistency, add to that any weight or dimensional difference, BC has little chance of being a constant that has any worthwhile benefit.
Having shot many different rifles over a 60 year association with them, I believe the only true way to set up and hit long range targets is with trial and error, perseverance aided by a little Strelok Pro on a still windless day recording the scope turret settings for future reference. I look at BC, weighing pellets, washing and resizing them, barrel harmonics, barrel cleaning etc and think what its all coming. If you believe all this is necessary then the days of loading your favourite pellet into your favourite rifle are numbered as you will need to check some computer app before proceeding.