Like the 1A, 2A isnt absolute, even conservatives say it isnt. You cant shoot guns for fun in a patio in NYC (but you should be allowed to own a mortar!). Not absolute but unconstitutionally stifled for decades.
You just have to pony up $200 for the mortar itself, and then $200 for each shell, to register them with ATF as destructive devices. Well, and find someone to willing to sell them for you. Or make your own.
Nevertheless, if you follow the procedure, it is legal, and rich collectors do it. Also applies to larger artillery pieces, tanks with working main gun etc.