pelletts gauges

Pelletgage was designed and first produced in 2015 for measuring the diameter of pellet heads. It is capable of determining head size within +/- 0.01 mm (0.0004 in) by a "go/no-go" process, with ten precision apertures cut into special sheets of fine grain stainless steel. It comes in .177, .20, .22, .25, and .30 calibers. Many users report immediate improvement. A few have used it to re-size heads, but this was not the design intent. It provides a way to determine the head size that works best for a given barrel, and then to assure that any new lot is within that range. For competition, shooters can sort out pellets within a narrow range easily.
It looks like this:

A google search will show many references. Tom Gaylord (AKA "BB Pelletier") wrote an extensive review for Firearms News last year, in which he tested the results of sorting pellets for different guns/pellets.
Gaylord, Tom. (2016, Jan) Pelletgage and the Speedy Pellet Inspector - New Tools that Can Improve Airgun Accuracy - Firearms News Vol 70 Number 3 (posted online by permission). I'll be happy to answer any question.