Human rights activists, dozens of whom protested outside the Stuttgart court, charge that G36 automatic rifles have been used in abuses including the 2014 disappearance and suspected massacre of 43 Mexican students.
In the dock are two former H&K directors, two former sales executives, one former sales staff and a former Mexico representative.
The six defendants, aged between 54 and 77, face maximum jail terms of up to five years if found guilty, and the court could also fine the company based in Oberndorf am Neckar, a major centre of the German weapons industry.
Germany is one of the world's top five arms exporters, along with the United States, Russia, China and France, and all its defence equipment sales abroad are subject to government approval.
Prosecutors charge that the 15 shipments of the military-style weapons between 2006 and 2009 breached Germany's so-called war weapons control law because they ended up in especially violence-torn Mexican states in breach of the export licence.
The Mexican defence ministry, which is in charge of gun imports, had approved the import of 9,652 H&K rifles, of which 4,796 went to states with particular human rights concerns, including Guerrero, German newspapers have reported.
Activists say that G36 rifles were also sent to police in Iguala, Guerrero state, where 43 students disappeared at the hands of corrupt police and were feared killed by a narco-gang in September 2014 in a case that sparked international condemnation.
A driving force in the investigation leading to the trial has been German rights activist Jürgen Grässlin, who first issued a criminal complaint against H&K staff over the Mexico sales in 2010.
“Those who enabled these weapons exports aided and abetted murder in Mexico,” he told regional public broadcaster SWR at the protest where demonstrators displayed pictures of disappeared students and G36 models splattered with fake blood.
The campaigner said it was well known that in the most conflict-torn Mexican states both police and narco-gangsters used the G36, and that “often the two groups cooperate”.
The court in Stuttgart has set 25 hearings until late October.