The vast majority of commercial actions, (factory), are machined from some type of chrome moly steel. That being, a variant of 4140. Those that offer stainless usually opt for 416.
There is a difference in custom actions machined from different stainless steel. It has evolved to where a manufacturer uses either 416 or a precipitation hardening stainless such as 17-4.
In my opinion, you can’t compare these two Stainless Steels. The truth being that all other things being equal, a 416 stainless action will not take the pressures that a action machined from properly heat treated 17-4. Action manufacturers such as BAT use 17-4, and do their own in house heat treating. This makes BAT one of the strongest actions available.
I have two of the early Farley Actions that are machined from 17-4. In particular, Jim used the lost wax ceramic mold method of casting these action bodies from 17-4. The actions are extremely stiff. Jim also machined his early bolts from S-7 tool steel with a relative high Rockwell C hardness. This gave the actions an ability to take the pressures encountered in Benchrest while having a good anti-gall quality.
There were a few problems. S-7 tool steel has great strength and impact qualities, but heat treat is extremely critical. To achieve the desired properties, an exact heat and time of heat is measured in very small temperature variations and time. Many of us can remember when Farley had a recall because bolt lugs were cracking. Myself and two friends took ours over to Lone Star Heat Treating and had ours tested. The RC hardness as all over the map. One of mine was on the upper end of what I would call safe, it was 53 RC. Never sent it back to have the temper redrawn, it has had thousands of rounds pas through it with no problems.
Why don’t all action manufacturers use 17-4? The main reason is cost in machinability. 416 at the desired strength level for an action body is much easier on tooling than 17-4. The strength level is adequate for the vast majority of rifle applications. Only in shooting applications where pressures climb into no mans land do you encounter problems, mainly in action stretching, resulting on difficult bolt opening.
Untill recently, I had an action on my Rail Gun that was machined from 416. My favorite 6PPC loads that shoot so well we’re just about unusable in my Rail Gun because in my Farleys, everything worked great. In the 416 action, a aggravating hard bolt lift was a constant problem with the same loads.
I solved this problem last year by going to a BAT Neuvo and a 17-4 action. I can now shoot the same upper end loads in both my Bag Guns and my Rail. BAT solves the galling problem by using a Melanite coating on the actions.
When an action manufacturer touts his actions as being machined from 416, I do not necessarily consider that a plus. An age hardening stainless such as 17-4 would be a better choice if ultimate strength as defined by tensil , yield, and ductility are considered.