Abstract : The M256 120-mm cannon is the main armament of the M1A1 and M1A2 tanks. With the increased pressure generated by the latest version of the M829 APFSDS-T cartridge (A3), the yield strength of the cannon would have been insufficient to prevent elastic deformation of the tube. Classic theory indicated that this pressure could not have been contained, but it was known that the theory is somewhat conservative. By testing to elastic deformation and using statistics analysis, a new Safe Maximum Pressure (SMP) as defined by NATO Standardization Agreement (STANAG) 4110, was determined. This resulted in the M256 tube yield strength being redefined and capable of firing the M829A3.