I’ve always thought the answer was ‘duh, obviously’ but I’m hard-pressed to find RAW evidence all of a sudden.
Assume you have a hand crossbow, loaded, in one hand, while your other hand is occupied (holding a sword or a very important relic or even missing for that matter). Can you fire that single shot from the crossbow before having to drop it for something more practical?
Ammunition Drawing the Ammunition from a Quiver, case, or other container is part of the Attack (you need a free hand to load a one-handed weapon).
Emphasis mine. That part might imply that the loading (and thus the hand requirement) is to happen as part of the attack.
Then again, when later referring to using a Loading weapon for a melee attack,
A sling must be loaded to deal any damage when used in this way.
This could imply the sling (and by extension other loading weapons) can be pre-loaded. Or that it needs proper loading, ammo etc. even as an improvised melee weapon.
Is there a solid RAW answer on this?