Aiming is still important obviously, but when you start training on these from the age of 6 years old it becomes just as accurate as the older non-atlatl versions
the device you're thinking about is called rifle grenade (soldiers already carry rifles with them)
sometimes 40mm UBGL fills the same role. grenade throwing device like this would probably require different fuzes (longer delay) and it will be much less controllable than anything actually launched (you don't want live grenades ending up nearby by accident, not an option with rifle grenades or 40mms)
the entire idea kinda died off because you can't fit in rifle grenade anything actually useful against tanks for example, but one resurgence would be 82mm mortars attached to PG-7 series rocket engine and launched from RPG-7, used by Ukrainians