WEIGHING BULLETS AND ACCURACY
“Accuracy”, here, is the region between .5” and 1.0” large sample 100 yard 5-shot groups.
When accuracy is important, cast bullets should be visually inspected and those with major defects put aside. Some of the remaining “good” bullets will have minor defects. The definition of “major” and “minor” is in the eye of the inspector. There is some evidence that smaller visual bullet defects do not cause inaccuracy.
Visually acceptable cast bullets are sometimes weighed. Weighing bullets identifies outliers and allows them to be separated. As an example, a certain set of bullets had the following weights, in grains:
52.9/1, 54.2/2, 54.8/9, 54.9/12, 55.0/55, 55.1/5.
The 52.9 and the two 54.2 grain bullets are obvious outliers.
If we assume that difference in bullet weight causes difference in height, elevation, of the bullet at the target, we might put these three bullets aside.
(Reasonable differences in bullet weight causes insignificant changes in velocity, and in elevation. See below.)
We assume that difference in bullet weight is caused by holes in the bullet, and, if we assume that these holes affect accuracy; then we might put these three bullets aside. (We do not assume that the density of bullet material varies.)
We’re left with these bullets:
54.8/9, 54.9/12, 55.0/55, 55.1/5
We could segregate these into four classes, or 3, or 2, or make them all one class. The fundamental question is: What is the relationship between variation in bullet weight and accuracy?
Well cast 200 grain bullet weights have a standard deviation of about .125 grains.
We would expect the average of a lot of batches of 100 to weigh about (R.E.):
199.7 1
199.8 5
199.9 16
200.0 58
200.1 16
200.2 5
200.3 1
And the fundamental question turns into:
What is the relationship between group size and the myriad of combinations of bullets of various weights? EX: 5 @ 199.8 vs. 5 @ 200.0, 5 @ 199.8 vs. 5 @ 200.2, 5 @ 199.9 vs. 5 @ 200.1…
Here is a method:
TESTING CARTRIDGE VARIATION AND ACCURACY
There are 2 furthest-apart holes that determine group size.
We wish to know if some variation in one cartridge, of five, increases group size with 5-shot groups.
Group size is the distance between the two furthest-apart holes in the target; call the holes A and B.
Call the hole made by the cartridge with variation, V.
The probability that A = V is .2; 20% of the groups will have A = V.
80% of the groups will not have A = V. This 80% has one of the holes = A; 1 of 4 = 25%; 25% of 80% of the groups has V = B.
The summed probability = 20% + (25% X 80%) = 40% of the groups shot will have either V = A or V = B.
If % A or B = V > 40, then the variation probably increases group size.
If % A or B = V ~ 40, then the variation probably does NOT increase group size.
If % A or B = V < 40, then the variation probably reduces group size.