A specimen, called the ring specimen, was developed to determine the fracture toughness Ktc, under conditions of autofrettage. Tests were carried out on standard arc shaped specimens and ring specimens without autofrettage to obtain Ktc values of a gun barrel steel chosen for study and seen to yield similar results. Tests were also performed on autofrettaged (100 percent overstrain) ring specimens at various crack lengths producing results with a dramatic decrease in Ktc of 60 per cent. From this, it may be inferred that the critical length of a crack in an autofrettaged gun barrel made of this material will be less than anticipated. Although cold work or autofrettage enhances fatigue life by decreasing crack propagation rate, it is seen to decrease Ktc creating, from this point of view, more dangerous conditions. For a design which must guarantee long life, the accompanying decrease in critical crack size may indeed prove a liability. Of course, this is a result for one particular material with a specific heat treatment and therefore not general; other materials must be tested, as well.