Cal .30 RANGE DUMMY cartridges were used by shooting coaches to detect and cure bad shooting habits such as flinching. The slot was cut into the base to identify the cartridge, while still resembling a live round.
left - M-1921, hs F A 21-R
center - M1, hs F A 30
right - M1, hs F A 30
Question. What is the pupose of the hole in the one on the right?