If a shotgun does NOT have a stock on it from the factory, then it is NOT designed to be fired from the shoulder, which is part of the legal definition of a shotgun.
Which is why the Mossberg shockwave and Remington tac 14 are legal and not NFA: they are longer that 26" (so not a pistol), and have never had a stock (so not designed to be fired from the shoulder).
They all (without stocks) are considered 'other' firearms. Because of the barrel length Maryland MSP considers the shorter ones as a handgun. The 18 inch barrel while still considered 'other' firearm by ATF, are transferred as a shotguns for anyone over 21. In other words the laws are very confusing. I suggest the OP check with ATF and MSP before breaking out the hacksaw. YMMV