They can always prosecute under "other weapons" in MGL C. 269 S. 10 . . . that allows them to claim that a fist, shoe, rolled-up newspaper, etc. is a weapon.Disclaimer - I don't live in Mass (does anyone there really live?).
Len, I remember it as slung shots, but this section mentions both?
General Law - Part IV, Title I, Chapter 269, Section 12
malegislature.gov
As I read it you can purchase or own any of these, but you can't manufacture them. Other laws probably limit carrying, like the "pistol" cane.
The way the weasels worded it, who actually knows what it means...
I don't recall seeing "sling shot" listed previously. That said, an inquiry many years ago yielded a response that the legislature can make "administrative" corrections in the law without hearings/bills/etc. So I suspect someone piped up that slung shots also meant sling shots, so let's add it without telling anyone.
I remember visiting Dick's in Attleboro back when C-pher worked there. They definitely had sling shots and accessories for sale on their shelves, as I gave it some thought but didn't buy them.