In 1956 , the T171 20mm gun was standardized by the U.S. Army and U.S. Air Force as the M61 20mm Vulcan aircraft gun .
After extensive testing , the T171 20mm gun was selected for further development .
In 1956 , the T171 20mm gun was standardized by the U.S. Army and U.S. Air Force as the T171 20mm T171 aircraft gun .
After extensive testing , the T171 20mm gun was selected for further development .