USCG-approved signal pistols (devices) are specifically exempt from being considered handguns under MD law (5.102(9)),
hence being able to buy them at the local Wal-Fart.
As for flare guns in general, well... technically they are considered handguns per 5.101(n)(2). Or NOT, per 5-132(4).
If you have an FFL-03 and buy from out of state, no worries either way.
<<<-- US M-8 37mm converted to fire 12 gyrojets, c.1965