http://www.globalsecurity.org/military/systems/ground/mk19.htm
They make them, it is a military only item...
So how do you get a "military only" item to smuggle into Mexico? You would need help from the GOVERMENT. You don't suppose the GOVERNMENT is leaking these weapons so they can be caught at the border ... therefore giving fuel and excuses for more gun control in the USA? (I wouldn't put them past it).