Originally Posted by goodgrouper
I have to disagree on several points. The 3 shot group has plenty of validity to it especially when you re-test that load again and it proves itself over. On magnum rifles with skinny barrels, this is pretty well the only method to use as the barrel heats up you can get bad data from a 5 shot group especially in warm weather. I have seen many people toss out "bad loads" with 5 shot groups because the last two shots went faster and in reality, the load could have been fine.
I also disagree that ES is a better measure of the uniformity of the load. The SD and ES are certainly related but the SD is a better "overall" indicator of the deviation from the norm just like it is named. Or in other words, the ES is the window of velocity but the SD is the variation found in that window.
If you re-test the load then you have effectively increased the sample size. For magnum loads or any load that heats the barrel too much I would just suggest waiting to let the barrel cool to the same point to prevent heat from changing the results.
I would not consider a sample of 3 to represent with any significant level of confidence the performance of a large number of rounds. You can get pretty deep into the statistics involved but to predict the performance of a large number of rounds, say 100, you need to test more than 3.
I am not saying SD is not a valid way to measure a loads consistency, simply that low ES is a tougher standard to meet, like using a 5 shot group instead of 3. My reason for using ES is mostly this; SD is a statistical way to predict deviation from the norm given a certain data set, ES is the actual observed velocity spread. I place more value on my actual measured numbers than on a prediction.
I don't mean to say that ES is the whole story, I suppose you could have a load with very consistent velocities but once in awhile throws one shot 100fps high or low...both the SD and ES might be fine, but I would not trust it.
The bottom line I think is you have to test and test and test to be sure that you won't get some unpleasant surprise on the shot that counts.