After reading Brian's thread about using the PelletGage some of the older guys who shoot can remember the first pellet gage. The BIC biro outer sleeve. In the early 90s we spent hours with the BIC sleeve and dropping pellets into it. Although the size was not known, the position the pellet dropped could be marked off and after a dozen pellets you had say three or four makes on the pen. Then as each pellet related to a mark the heads were either big or small.
To remove the pellet a sharp tap on the table and the pellet dropped out ready for the next. Nick JENKINSON done his like that for many of his rein as top FT shooter.