Stainless steels, especially the 300 series austenitic steels, are more likely to be chosen for their corrosion resistance than their strength. Tensile is usually in the 65 kpsi range, although 430 stainless does hit 95 kpsi. A lot of good guns have been built with 400 series stainless.
SAE 4140 is a normal modern choice for gunmaking steels, has good machineability and the tensile strength at 95 kpsi is sufficient to allow fairly light weight.
If you want really strong stuff, go to the maraging steels that Remington uses for their flush Full steel rated choke tube. Tensile up to 510 kpsi and yield up to 351 kpsi. I had never heard of them before RemChokes, but I have since had several extreme duty industrial parts made out of it with excellent success. THAT is tough stuff.
