In france, there's four categories of weapons, rated from A (highly restrected) to D. The sling would definely go as a "C-class weapon" as a propeller shooting over 20joules. Well... Sling is no propeller, merely a lever, and almost every adult human can easily throw a rock with KE over 20j, with or vithout lever... Nah, go and try to explain this to a juge...
legaly, you can own a sling, providing you had no problem with the police before, and you are a registered hunter, or registered sportsman, but you have to register it to the "prefecture de police", witch is a fairly high instance of the police. you cannot go with it, unless not ready to use (a bow must be unstringd; should a sling be unbraided?)
at home it must be secured in a safe or another adapted box.
and if you've been naughty, you risk 2 years of jail, and a 30 000€ fine, up to 7 years and 100 000€ per person if you are - or look like - an organised group.
Well, obviously this nonsense was intended for firearms, and was expanded to include any type of weapon, with little reflexion...
Never heard of someone beeing jailed for sport- or play slinging, but given the justice ways in france...
references:
https://www.legifrance.gouv.fr/affichCode.do?idSectionTA=LEGISCTA000027591634&ci...https://www.legifrance.gouv.fr/affichCode.do?idSectionTA=LEGISCTA000029658835&ci...