Now load that with a powder measure that varies +/- 0.4gr with Varget (meaning dropped charges span a range of 0.8gr)
Wow. If your powder measure is really that bad, we ought to chip in and treat you to a new one that is at least mediocre.
With Varget, my Lee powder measure has a standard deviation of .11 grains, measured. So 95% of charges will fall between plus and minus .22 grains, and the math works out as shown.
As stated, stick powder is a worst case condition, and so is a small case. The laws of physics being what they are, ball powders and larger cases will turn in more favorable results.
But just for grins.....
Assume that you are measuring powder down to 1/10th of a granule, and that variation in bullet weight, neck tension, case capacity, etc. are giving you a standard deviation of 30 FPS. So you switch from measuring down to 1/10th of a granule to using my powder measure. Here are the actual numbers:
223/5.56, Varget. The standard deviation of 30 FPS becomes 31.95 FPS as previously shown.
223/5.56, ball powder. SD of the powder measure is .041 grains. The standard deviation of 30 FPS becomes 30.28 FPS.
308, Varget. The standard deviation of 30 FPS becomes 30.67 FPS.
308, ball powder. The standard deviation of 30 FPS becomes 30.04 FPS.
I've given you all the necessary steps, so you can make the same calculation for other cartridges if you like.
