Broad recognition for a comparatively straightforward application, and it fills gaps in the mid-Atlantic that the New England licenses don't reach.
Issued by a Pennsylvania county sheriff's office that accepts non-resident applications. Massachusetts residents apply as non-residents — you do not need to live in Pennsylvania.
The 39 State Multi License Course teaches the law for Massachusetts, Maine, New Hampshire, Connecticut, Pennsylvania and Florida in one sitting as the Massachusetts-only class. You then apply for whichever of the six you actually want.

Six states' course requirements, in one visit, for $349 — instead of $1,249+.
Elsewhere on the site: Getting licensed · Massachusetts gun law · Your town · At the range · Who it's for