Thread: Stupid Question. bare with me please

Originally Posted by gimp

Agreed. What does a 3 shot group tell us about ES?


Well let's look at the ES of 15 different groups (ranging from 7-20 shots) vs the ES you get from the first 3 shots in the group. It looks like this.


Attachment 256680

The first 3 shots aren't very representative of the (more true) ES that appears with a larger sample in the majority of cases.


we can also look at a sample of 39 shots with the same rifle/load, sampled artificially as "3 shot groups" and see how representative of the true ES 3 shot groups are. It turns out that they are wildly variable; and none come close to demonstrating the "truer" ES measured across all shots.


Attachment 256678


However ES is quite a poor measure of how consistent our velocity is and how consistent we can expect it to be in future. It only takes into account the 2 most widely separated values and ignores the values of all other shots. Standard deviation is a better tool. Depending on the variability of your velocity, you should expect to get "reasonably" close to the true SD value across a sample of 10-15 rounds. Standard deviation is what should be used to assess hit probability.

4x your standard deviation should be the range in which 95% of your velocities can be expected to fall. Note that I seem to have a sample size where that is currently true. I would expect 99.7% of my shots to fall within a range of 6x my SD - i.e. about 115-120fps "REAL ES". Note this is a .223 and every .223 load that I've collected sufficient data on, across several different rifles, brass, bullets and powders, loaded by different people, seems to end up with an SD around 15-20.

3 shot groups do not give you a good estimate of your SD. The standard practise of shooting 3-shot groups at different charge weights for load development and looking for a low value is invalid.

It can look like this:
Attachment 256679

You might conclude that load number 4 is the best load based on the consistent SD. There's a "node" there, right?

Wrong - every group on this chart is shot with the same load. The 3-shot SDs tell you essentially nothing.

Not to turn this into a discussion on velocity consistency, but I'll round my thoughts off by noting that hunters shouldn't be terrible concerned about it (at all).

At ranges out to 400 metres, the hit probability difference between an SD of 10 and an SD of 30 (i.e. a "real" 95%ES of ~40 vs ~120) is essentially negligible. 100% vs 98% hit probability on a 200mm dia circle in perfect conditions.

Hunters would be far better served to prioritise their time away from the reloading bench - rather than trying to "tune" the load via methods that probably don't do anything in reality - learn to shoot from positions and read the wind. That's more likely to improve hit probability by a significant amount.

Set requirements for your load based on an understanding of hit probability, find something that meets those, then proceed to using it.