Four of the victims in the daylight attack were believed to have been peddling drugs from the store in the industrial city, said Monterrey public security spokesman Jorge Domene.
Two other unidentified victims, a man and a woman, were likely bystanders who were in the store when the gunmen got out of an SUV and sent a barrage of gunfire into the establishment to the west of the city's center.
He said the violence was the result of a turf dispute between drug gangs.
Domene also confirmed that the killing of three brothers at a taco stall in Monterrey earlier Monday was likely linked to a rivalry between drug gangs, bringing to 40 the number of killings in the city related to organized crime in 2011.
Drug cartel turf battles around the country have left more than 30,000 people dead since December 2006.
Mexico City has been spared most of the violence, but police said eight people died in a shootout among drug dealers Sunday in the La Esperanza neighborhood of the city of Nezahualcoyotl, a suburb of the metropolis.