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Originally Posted by tangolima
I have seen promising 3-shot groups turned out to be dud after firing 7 more rounds, and vice versa lousy ones turn good. Here I don't take extreme spread, but mean radius calculated.
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Take any measurement you want. Take a box of .22LR -- 50 rounds to a box. The advertised velocity for CCI standard velocity .22LR is 1,070 fps. Does every round in a 50-round box produce 1,070 fps through whatever barrel they used as a test barrel? Of course not. I have no idea what their acceptable deviation is, but call it +/- 5%. That would mean that for each 50-round box they grabbed 50 rounds out of a production batch of many thousands, and any one of those rounds (each one of those rounds) could have a velocity anywhere from 1,017 fps to 1,124 fps. One box may, by pure luck of the draw, get a high percentage of "fast" rounds, and the next box may get a high percentage of "slow" rounds.
The fewer rounds you test out of that box, the less chance there is of getting a sample that accurately represents the true range for the entire batch that you're sampling. In reality, out of a day's production of .22LR even an entire box of 50 is likely not going to hit an average velocity of 1,070 fps. And if it does -- is it repeatable? In other words, if you pull three or five boxes out a production run and average each box as a sample set of 50 rounds -- would any of them average 1,070 fps? How close would the average of the three or five samples be?