 In practice, due to policy and practical reasons, aircraft were closing to visual range and maneuvering, placing the larger US aircraft at a disadvantage to the much less expensive day fighters such as the MiG-21.Missiles proved to be much less reliable than predicted, especially at close range.Although improved training and the introduction of the M61 Vulcan cannon on the F-4 did much to address the disparity, these early outcomes led to considerable re-evaluation of the 1963 Project Forecast doctrine.