J. Magness, Masada (2019), p. 11: lots of slingstones found in the Roman camp F, in heaps next to the individual tents of the 8 men units. At least that's what she things the big round pebbles specially brought there (and not removed when camp evacuated).

If true, I suppose the idea is that Roman legionaries had, in camp, ammo dumps next to each tent (=8 men). In case of alarm, when running to arms, every man could if need be, grab at least one big smooth stone, and if need be, throw it or sling it at attackers

Lots of arrow heads found int he siegeworks but not slingstones, I think. No ballista bolts either but Magness thinks the Roman collected them (just like they collected arrows)