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I still see a lot of people wondering why they fail to meet PF at a match. If you just add a few extra power factor points, now you know what can happen. The bottom line is that velocity is random in nature and the BEST way to understand and manage it is to use statistics. You don’t need to have any special math skills. This post provides the simple steps to follow. For the why and more details, see Frontsight Magazine, Jan/Feb 2015 Edition, Pg. 70. The standard deviation (STD) measures the amount of variation (uncertainty) around the average velocity and it should always be used when reloading. The table below maps the chance of failing an official USPSA PF check to the variable Z. Use Z for reloading by simply multiplying the value of Z by your STD (measured using 8 rounds with your chronograph) and add the result to the required velocity to make major or minor PF for your bullet weight. The result is your desired load velocity. Values of Z are shown from 0 to 3 to illustrate the tradeoff between Z and the chance of failure. My guidance is to use values of Z in the 2 to 2.5 range. Note that if you use the STD (i.e., Z=1), which I’m sure some people do, the chance of failing PF is a whopping 44%. Z Chance of failing PF (per USPSA rules) 3.0 3% 2.5 5% 2.0 10% 1.5 21% 1.0 44% Assume a shooter’s standard deviation for a load with a 155 gain bullet is 15 ft/second (chronographed using 8 rounds). An average velocity of at least 1065 ft/sec is needed to make major power factor. To limit the risk of failure to no more than about 10%, use Z = 2.0 and simply add 30 ft/sec (Z*STD=2*15) to the required velocity to meet PF. Thus the desired load velocity would be 1065+30 =1095 ft/sec. It’s that easy! In our example above, if we “just add a few PF points” and loaded to say 168 PF, this is equivalent to Z=1.26 and the result is a 1 out of 3 or 33% chance of failure. If instead we use the measured extreme spread (which can typically run 60 fps or higher) and load to 1065+60 =1125 fps, it is equivalent to 4*STD or a 186 PF and we needlessly incur excessive recoil and higher cost. If you always load to say a PF=170 (1097 fps), your chance of passing is okay so long as your STD remains below 16 fps (1097-1065)/2. However, if your STD is actually 23 fps, Z drops from just above 2 down to 1.4 (i.e., 32/23) and your chance of failure more than doubles going from below 10% up to 26%. This is why you should measure and use the STD. Shoot with any comments or questions!
