I think a good question would be how soft is too soft. Rifle barrels of 4140 for high pressure cartridges are typically listed as having a rockwell C hardness in the mid twenties. It may not even be possible to treat 4140 to be much softer than that.
All true; Of course if rifle for high pressure cartridges happens to be a bolt action if the steel for the bolt lugs & their seats in the receiver were of that same mid-twenties Rc hardness it would be only a few shots until it had developed Dangerously Excessive head space. Both the steel & its treatments need to be selected according to use.
I will simply say, that to the best of my knowledge the older doubles made of low carbon steel were "Proofed" In The White. That even in this form they were adequately strong is evidenced by the fact they passed. They were however "Not" left in this condition & for a reason other than just the colors. "IF" I had a frame made of 4140 as long as it was in my possession it would "NOT" intentionally be exposed to temps in the 1200°F range. Nuff Said.
