Complex sentence: The end result is an inner surface of the gun barrel with a residual compressive stress able to counterbalance the tensile stress that would be induced when the gun is discharged that in addition has a higher tensile stress as a result of the heat treatment process.
Split into simpler sentences: The end result is an inner surface of the gun barrel with a residual compressive stress able to counterbalance the tensile stress that would be induced when the gun is discharged. In addition the material has a higher tensile stress as a result of the heat treatment process.