Thread: Drop chart help

I'm going to disagree with the above posts in this situation for you James, and say that IF you can get a muzzle velocity reading using either a Labradar or a MagnetoSpeed chronograph, averaged over a reasonable number of shots for your load, then the calculated drops from a trajectory calculator are almost certain to be a better predictor of where your future shots are going to land than from using the measured drops recorded on the particular day and at the location that you take them, at least out to 500 metres if not further. However if you don't use one of the above chronographs then the accuracy of the average muzzle velocity readings could well be insufficient to support this statement, as most other chronographs are… well, not suitable would be the nice way of putting this.

I'm not going to leave this as an unsupported opinion, so the logic behind the above statement is this:
There is an assumption in using the actual recorded drops that,

1. the groups you have shot at your zero distance have allowed you to correctly set the zero, inasmuch as a sufficient number of shots have been taken, and the group size is sufficiently small. This is important as the error in the zero will compromise all the drop figures that you go on to measure, and,

2. the same as above for all the groups shot for drops; sufficient shots will be taken at each distance and result in small groups, so that the observed group centres have a reasonable probability of reflecting the true average point of impact.

We know that the average hunting rifle is doing well to shoot 1moa with preferred match ammunition, so at least a 3.5” group at 300m in the best of conditions. A shooter would then need to layer on the variation added by the ability to place those shots, and on this aspect the internet in general is filled with reports and expectations that are completely at odds with my observations. Then, if as is likely, these groups would be formed from just three shots each, the deviation from the centre of these groups from the population centre has a high probability of being quite some way off.

The only parameter that the difference in predicted drops from a ballistics calculator and the actual drops you would measure is going to identify, is a correction to the published ballistic coefficient of the bullet you are shooting — nothing else. In effect, if you measure and use your drops rather than the predicted figures, you are saying that your data is of sufficient quality to make a correction to the manufacturer’s published BC. It is only at much longer ranges than you are considering for your drop table, that deviations in BC due to batch variation, stabilization factor, and deviation from the G1 or G7 models being used become apparent on the target out of the noise, and even then only for very good shooting and when all atmospheric and situational factors have been incorporated.

The argument that recording actual drops to create a lookup table, will automatically better take into account environmental and situational conditions than a trajectory calculator, is also wrong. When recording actual drops to form a lookup table, you will be locking in the environmental and situational conditions of the day and at your shooting location. For example the range at Tauranga might be close to sea level whereas your average altitude for hunting might be 700 metres. Better to let a calculator figure out the trajectory from having entered mid-range inputs for the all the variables appropriate to your typical hunting situation than end up with a table of data that is badly skewed for your actual application.

In my earlier post I mentioned that actual drops could be use to verify those predicted by a ballistic calculator. The reality will be that your actual drops if/when you come to record them will be different from those predicted by a trajectory calculator. The problem will then become: which do you believe?