No problem on the length since the 91/93 receivers are longer than MP5 receivers. Using a longer side folding UMP stock puts it right at 29" and a hair. An A2 and A3 stock puts it at 28" unless I wanted to weld on the suppressor mount. Or I could make it a pistol and only use my sear with a stock but the C93P isn't on the list. Its twin the Vector V53 is though.
Only the HK SP-89 is on the MD banned "assault pistol" list. So you can have any 91/92/93 based pistol (that you can get on the roster). Can SBR them if you can reach 29".