Defence lawyers persuaded the court on Wednesday that David Hudak had not acted unlawfully when purchasing explosives, despite his paranoia over security threats since the 11 September 2001 attacks.
The government had alleged 42-year-old had purchased 2400 warheads that could be used as armour-piercing missiles.
But it transpired the defendant purchased the explosive devices through Halliburton, the Houston-based oilfield services company that was once led by US Vice President Dick Cheney.
Hudak, president of High Energy Access Tools Corp (HEAT) in Roswell, New Mexico, was found innocent of all nine US weapons charges against him.
He could have faced up to 30 years in prison if found guilty, but it took a jury less than a day to dismiss the charges after a trial that lasted about six weeks.
Defence lawyer Bob Gorence argued the explosives were purchased legally, with all the paperwork properly completed.
A spokesman for Halliburton also confirmed a subsidiary sold demolition devices to the school but that it did not sell warheads.
However the defendant is still in jail on charges of violating his visa to stay in the United States.
He has been in jail since August 2002 when he was arrested and charged with illegally possessing the massive cache of weapons after federal authorities raided his school.
HEAT began to worry law enforcement agencies when they learned the school had clients from Yemen and the United Arab Emirates.
At the time of his arrest, Hudak was deemed by a court as a flight risk and denied bail and the school head will return to court later on immigration violation charges.