In France, the legal status of weapons is defined by the
"Décret n°95-589 du 6 mai 1995 relatif à l'application du décret du 18 avril 1939 fixant le régime des matériels de guerre, armes et munitions" (concerning war materials, weapons and ammunitions) (125 articles...).
Weapons are classified in 8 different categories and the sling can be put under the 6th,
Art. 2 §1 :
"6e catégorie : Armes blanches.
Paragraphe 1 : Tous objets susceptibles de constituer une arme dangereuse pour la sécurité publique, et notamment les baïonnettes, sabres-baïonnettes, poignards, couteaux-poignards, matraques, casse-tête, cannes à épées, cannes plombées et ferrées, sauf celles qui ne sont ferrées qu'à un bout, arbalètes, fléaux japonais, étoiles de jets, coups de poing américains, lance-pierres de compétition, projecteurs hypodermiques."
Anything that may be a dangerous weapon for public safety, including [...] crossbow, nunchuk, shurikens, knuckle dusters, competition slingshot[...]
The
Art 46-1 state that you are free to buy and possess any weapon of the 6th category if you are over 18.
If you are a minor, you can't buy a sling (or a sword cane). But you can have one if you are over 16, your parent (or equivalent) authorised it AND you are a licenced of a sport federation having an official ministerial accreditation.
BUT, according to the
Art 57 you cannot carry them without "legitimate reasons". One of those reason seems to be the practice of a sport, in a federation accredited. You also have to carry them in such a way they can't be easily used, either by using a security system or by taking it apart (or at least an important part).
So slings are legal to buy and own, maybe not to carry and not if you carry them with rocks or lead ammo. Of course, you can't be sent to jail for having shoelaces, but...