A quick search turned up very little. But it is illegal to discharge a firearm, even if you can carry it. I found this on a Fenway Park site, so I assume it governs the ball park as well as other Boston parks.
SECTION 2. No person shall, in any public park (including any boundary road thereof), or other public place (including any parkway) under the control of the Parks and Recreation Commission, except under the auspices of public authority:
(a) sit, stand or lie upon, or climb upon or over, any balustrade, railing, fence, wall, roof, statue, monument, fountain, bush or tree; or
(b) go under any balustrade, railing or fence, or
(c) stand or lie upon any seat; or
(d) go upon any flower bed or cultivated area; or
(e) dig up, cut, break, remove, deface, defile, or take any tree, bush, plant, turf, rock, gravel, building, structure, fence, railing, sign or other thing connected with such park of place; or
(f) disturb any bird’s nest or eggs; or
(g) injure or have possession of any wild animal or bird; or
(h) set any trap or snare, or
(i) throw any stone or other missile; or
(j) drop or place and suffer to remain a lighted match, cigarette, cigar or other burning substance; or
(k) have or discharge any firecracker or fireworks; or
(l) discharge any firearm or destructive weapon; or
(m) have in any such park or place, except a boundary road or parkway on which there are dwellings, any firearm or destructive weapon; or
(n) go on foot on any drive or bridle path except to go directly across from the walk to another; or
(o) sit, stand or lie upon lawn areas in the Public Garden except such lawn areas designated as sitting areas.
