Steel similar to SAE 1018/1020 (about the lowest strength for steel) has been used successfully for firearms frames since "the beginning." The advantage of higher strength steel is that a lighter weight frame can have "the same" strength. However, since the modulus for steel (length of strain (movement) for a given load (tension/compression/shear) is basically uniform across the sweep of alloys (some stainless is a little lower). So, if one plans to take advantage of an alloy's higher strength, one must design for more relative movement among the lever work parts. Substituting a higher strength alloy will improve fatigue as long as "notch fracture" characteristics are properly allowed for. I'm not sure how gunmakers and designers view steel alloy selection, but it is for sure that they have a plethora, a pleather, I say, of choices in this day and age.
DDA
