Sorry for late response. Basically it was only available to be unlocked universally in MW2 if you completed the DMZ story mission "Boating and Entering" back in Season 2 and 3 as it was a blueprint reward. You can no longer get it as a blueprint, however you can have it as a contraband weapon in your stash.
Make sure it's Hard Mode, and do "End Product" mission instead because the challenges are bugged. Doing "End Product" will check off "Make Ends Meat" instead.