Juan Erquiaga was a Peruvian Army officer who was introduced to Gordon Ingram and the Police Ordnance Company, probably during Ingram's time working on sales of the Model 6 submachine gun to Peru. Erquiaga first moved to the United States in 1951, and was hired by Police Ordnance. During his time working there, he began working secretly on behalf of Fidel Castro's Cuban rebels working to overthrow the Batista regime. The would lead to his hasty departure to Mexico just ahead of law enforcement, where he continued working on arms design and supporting the Cuban rebels. When they successfully overthrew Batista, Erquiaga moved to the island to work as an armorer for Castro. This was a short-lived situation, though, as Castro ultimately opted to use Soviet military aid instead of building a domestic arms industry, and Erquiaga defected back to the United States in 1961.

He set up a gunsmithing shop in California, and one of his first products was the EM-62, a conversion of the M1 Garand to fire 7.62mm NATO from M14 or FAL box magazines. These were sold on the US civilian market in both semiauto and full auto variations (although thanks to the NFA tax, very few EMFA-62 automatic types were sold), but the real goal was a military contract. Not one with the United States, but rather with a country like Peru or Taiwan that had substantial numbers of surplus Garand that they might prefer to update than replace. These hopes never bore fruit, though, and Erquiaga moved on to making a sketchy Sten knockoff submachine gun for Costa Rica, designated the MR-64. He came back into contact with Gordon Ingram, and Ingram was working for him at the Erquiaga Arms Co when the MR-64 project came to the attention of Federal law enforcement, and all of the guns were confiscated. These included Ingram's prototype M10 and M11 guns.

Erquiaga fled back to Peru in 1965, and Ingram moved on to work with Sionics and finish development of the guns that would be most closely associated with his legacy, the MAC M10 and M11.