How to measure odd grove bullet diameter
The V-anvil micrometer is the best way but it is an expensive and specialized tool that most people won't want to pay for, especially to measure only a few slugs. A way to simulate one is to make a small V-block of the appropriate included angle (108° for five grooves), calibrate it with an accurate diameter pin, and then measure the bullet and V-block with an ordinary micrometer. Then do the math given below.
The math goes like this:
Let a = the included angle of the V-block,
t = the thickness of the V-block from the bottom of the V to the bottom,
h = the measured height of the bullet and V-block,
and d = the diameter of the bullet.
Then d = 2(h - t)/(1 + 1/sin (a/2) )
An example:
For a 5 groove bullet, the included angle (a) is 108°, assume the V-block "thickness" (t) is .250" and the measured total height (h) is .5854", then
d = 2(.5854 - .250)/ (1 + 1/sin (108/2))
d = 2(.5854 - .250)/ (1 + 1/sin 54)
d = 2(.5854 - .250)/ (1 + 1/.80901)
d = 2(.5854 - .250)/ (1 + 1.23607)
d = 2(.5854 - .250)/ (2.23607)
d = 2(.3354)/(2.23607)
d = .6708/2.23607
d = .300
The easiest way to determine the V-block thickness (t) is to measure a known cylinder and then calculate it from
t = h - (d/2) (1+1/sin(a/2) )
With typical groove depths, you should be able to measure both land and groove diameters if the slug isn't too long.
Note: this process will work with a 90° V-block (using 90° for the included angle) and a 5 groove slug for groove diameter if you are careful to position the slug with the large diameters on the slopes of the V. You won't be able to measure the small diameter though.