There is no way to remove or otherwise manipulate a single specific magazine in a container.
Which is a bit of a pita for say magazine repack stuff, and such.

So was wondering whether it could be possible to have this kind of functionality by using the ids returned with magazinesDetail (edit: oh, and with magazinesAmmoFull)? 🤔
Basing this on the fact that there is a unfinished/unsupported-ish command removeWeaponCargo with that kind of functionality for weapons (same kind of command exists for weaponAttachments too).

_container removeMagazine [<id>, <cr>];

To go even further would not mind at all if something like setMagazineAmmoCargo could be added (or alt syntax fro setAmmoCargo):

_container setMagazineAmmoCargo [<id>, <cr>, <ammo count>];