Extreme Velocity Spread and SD have always made me space out...
In the user’s manual for the CED M2 chronograph, they say the following:
statistically, it has been proven that 68% of all things measured will fall between one
standard deviation above or below the average. Additionally, 95.4% will fall between two standard
deviations above or below, and 99.7% will fall between three standard deviations above or below the
Let’s look at an example.
Average = 3000 fps
Spread = 9 fps
SD = 5
Just about all would agree that’s a pretty good load!
Let’s say we shoot the rifle many times and over all it shoots with the above mentioned consistency.
Now, by definition,
68% of the shots would fall between 2995 fps and 3005 fps.
95.4% of shots fired would fall between 2990 and 3010 fps.
99.7% of shots would fall between 2985 and 3015 fps.
This would also mean that if we shoot a group of 10 shots and we know for a fact that our load has a SD=5 then:
About 6.8 shots would have a velocity between 2995 fps and 3005 fps.
9.5 shots would fall between 2990 and 3010 fps.
9.97 shots would fall between 2985 and 3015 fps.
This insures me that from my average velocity all shots would be somewhere between ±15 fps.
Bottom line, if I’m shooting my 300 RUM light load using 210 gr. Berger at 3000 fps muzzle velocity, when my mv=3015 it’d be 2.0” high and when mv=2985 fps it’d be 3.0” low.
My conclusion is that if we want to shoot ½ MOA at a 1000 yd, we’d need our load to be in the single digits for ES and SD.
What do you guys think?