The bell curve is actually showing a frequency distribution. In other words, how frequently certain numbers appear in relation to the average. For example, suppose the average in Figure 1 is a muzzle velocity of 3000 fps, and you measure an SD of 10 fps. The bell curve predicts that 67% of your shots will fall between 2990 fps (the average minus 1 SD), and 3010 fps (the average +1 SD). Furthermore, you can see that 95% of your shots should fall between 2980 fps (the average – 2 SD’s) and 3020 fps (the average +2 SD’s).