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?

It appears, since the hole is in the groove, to be just another proof of being inert, perhaps not done originally but later, by some collector or other person using the cartidge for decorative purposes.

If the slot was in the rim, the hole would probably indicate a “copper crush” pressure test round, but I don’t see it having been that at any time in the cases history, since evidently their is no alignment notch in the rim.