IANAL, nor 2A case law buff...but given the Troubles of 7 February 1991 ( https://en.wikipedia.org/wiki/Downing_Street_mortar_attack ) - and much other mischief that they'd be "just the thing" for - I suspect that U.S. law enforcement could be very interested in private owners of mortars.
That's partly why they set it up that way in the first place - ATF gets to review all those applications, and while they cannot reject them except for specific enumerated reasons, they can certainly investigate and/or notify other agencies.
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.
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.