Got a ref.? I ask because the only places I see "slingshot" as illegal are listed in the same list as metal knuckles, billy clubs, and automatic knives. In these cases, my research indicates that the actual weapon in question is what we now call a "Slungshot." This is a simple flail consisting of a weight at the end of a flexible line or cord. It encompassed everything from the classic Sailor's braided Slungshot to a rock in a sock. My research shows fairly conclusively that the i/u spellings were used interchangeably prior to the 1930's or so and in news reports and "common communication" could easily refer to either device. You'd have to figure out by context which was meant. However, almost universally, the flail was associated with violence, either as a self defense tool or, much more often, as a criminal weapon. The projectile firing tool is more often associated with juvenile mischief, like killing birds and breaking windows, than "serious" criminal misdeeds (though there was a very interesting reference to New York Suffragettes using "slungshots" to sling lead tokens imprinted with slogans through shop windows).
Though I'm still slogging through modern States laws and their myriad revisions, every time I come across "slingshot" in the criminal code, it's sandwiched in with Billy Clubs, Truncheons, Sand Clubs, Saps, "Slappers," Spring-Clubs, and similar, all right beside brass knuckles, and automatic/gravity knives. This makes it clear to me that the Slingshot being referenced is the inexpensive criminal bludgeoning flail instead of the robin killing, window breaking, stone-flinging tool.
If you have a reference to slingshots being illegal and it's clear that it actually means the modern understanding of a slingshot, I'd love to see the State Code, please. I'll even say pretty please if it'll help.
Peace favor your sword,
Kirk