Yes, sling shots ARE legal(to possess) in Massachusetts

Sling shots are commonly considered to be illegal to possess in Massachusetts. The law commonly cited is section 12 https://malegislature.gov/Laws/GeneralLaws/PartIV/TitleI/Chapter269/Section12

“Whoever manufactures or causes to be manufactured, or sells or exposes for sale, an instrument or weapon of the kind usually known as a [long list, not going to post it all here] slung shot, sling shot [list continues],shall be punished by a fine of not less than fifty nor more than one thousand dollars or by imprisonment for not more than six months; provided, however, that sling shots may be manufactured and sold to clubs or associations conducting sporting events where such sling shots are used.”

Note the word “possess” is missing from this.
A listing of weapons where possession are specifically banned is listed on Section 10
https://malegislature.gov/Laws/GeneralLaws/PartIV/TitleI/Chapter269/section10

“(b) Whoever, except as provided by law, carries on his person, or carries on his person or under his control in a vehicle, any stiletto, dagger or a device or case which enables a knife with a locking blade to be drawn at a locked position, any ballistic knife, or any knife with a detachable blade capable of being propelled by any mechanism, dirk knife, any knife having a double-edged blade, or a switch knife, or any knife having an automatic spring release device by which the blade is released from the handle, having a blade of over one and one-half inches, or a slung shot, blowgun, blackjack, metallic knuckles or knuckles of any substance which could be put to the same use with the same or similar effect as metallic knuckles, nunchaku, zoobow, also known as klackers or kung fu sticks, or any similar weapon consisting of two sticks of wood, plastic or metal connected at one end by a length of rope, chain, wire or leather, a shuriken or any similar pointed starlike object intended to injure a person when thrown, or any armband, made with leather which has metallic spikes, points or studs or any similar device made from any other substance or a cestus or similar material weighted with metal or other substance and worn on the hand, or a manrikigusari or similar length of chain having weighted ends”

This part of the law DOES mention “slung shot” which causes confusion for many. A slung shot is NOT a sling shot.
From Wikipedia https://en.wikipedia.org/wiki/Slungshot

“A slungshot is a maritime tool consisting of a weight, or "shot", affixed to the end of a long cord often by being wound into the center of a knot called a "monkey's fist". It is used to cast line from one location to another, often mooring line. The cord end is tied to the heavier line and the weighted end of the slungshot is thrown across the intervening space where a person picks it up and pulls the line across.

As a weapon
The slungshot was often used as a civilian or improvised weapon; however, the rope was much shorter for use as a weapon. The cord is tied around the wrist, and the weight is carried in the hand or the pocket of the user. A slungshot may be swung in a manner similar to that of a flail or a blackjack.”

This doesn’t mean that knocking over some tin cans in a back yard or woods comes with no risk. A cop could easily misinterpret a “slung shot” as meaning “sling shot” and it’s off you go to the liberal judge. Always remember, the law is whatever the judge tells you it is. So yes, you could still get jammed up, or at the very least get stuck paying out big money to lawyers due to a cop’s misinterpretation of this law.