The aggressive environments experienced by large-caliber gun tubes during processing and firing have led to much investigation of the hydrogen-induced cracking susceptibility of high-strength steels. Constant displacement bolt-loaded specimens were used to determine the hydrogen crack growth rates and threshold stress intensities of AF1410, both conventionally and isothermally heat treated, and AerMet 100. The severe susceptibility of high-strength steels necessitated the application and modification of a low-cost, highly reliable in-situ crack measurement method, the instrumented bolt. The instrumented bolt consisted of a full bridge, strain-gaged stainless steel bolt coupled to an automatic data acquisition system. New expressions were developed for use with the instrumented bolt and bolt-loaded specimen to relate load to crack growth. Stage II crack growth rates for the AF1410 were 1.1 × 10-2 and 2.3 × 10-2 mm/s for conventional and isothermal AF1410, respectively. Threshold stress intensity levels for AF1410 were 16.0 and 13.7 MPa ∙ m1/2, respectively. Stage II crack growth rates for AerMet 100 were 2.4 × 10-2 mm/s, while the threshold stress intensity was 14.1 MPa ∙ m1/2.