Specialty Steels: GBQ, CrMoV, 416R, 4140R, 4150R
Summit Steel brings its same high quality services and solutions to firearms manufacturers, offering the following grades: GBQ steel. Gun barrel quality steel. Chrome Moly steel. SBQ-AQ-BQ-GBQ | MIL-B-11595E (AR)
For a Mil-Spec barrel steel; the specification would be listed as MIL-B-11595E. This is to say that not all 4150 steels are considered Mil-Spec. In order for it to be Mil-Spec it would need to be a 4150 grade AND it would have to comply with the specifications listed in 11595.
An 11595 barrel steel (Mil-Spec) gets its additional strengthening properties over the common commercial 4140 by one of two ways;
more carbon in the steel
more carbon with the addition of Vanadium.