was wondering if it is legal to carry a flare gun on your person
In all states I am aware of (I am licensed only in Virginia) the definition of a firearm turns on the presence of a barrel, a projectile, and the propellant. A flare gun has all the requisite basics to be considered a firearm. Although you should run down your states' definition online easily enough, I suspect you will find it IS a firearm.
Whether YOU can carry a firearm depends on your state's laws regarding carry, conceal and carry, and whehter or not you have any disabilities that would prevent you from possessing a firearm (felony convictions, history of mental issues, DV, etc.).
Best option is for you to consult with a MA attorney directly for a read on what your state allows and what your specific situation is.
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Admiralty / Maritime Attorney
A flare gun is considered a firearm. You can carry it on your person, at least on land, under he same conditions and circumstances as you could carry a revolver or semi-automatic pistol.
On a vessel, however, in addition to being a firearm, it may also be a required piece of safety equipment. Consider your standard, run-of-the-mill, 12-gauge flare gun (see link below). It is approved by the USCG (the Department of Homeland Security!) as required safety equipment, but it is also a firearm and, in many states, meets the definition of a illegal "short-barrelled shotgun." If you carry it aboard your boat on navigable waters, there is almost a certainty that you will not have a problem. Get caught carrying the same flare gun under your shirt in a convenience store, and you had better have either a permit or a good criminal defense attorney..