This is intended to be a companion thread to my other one. This has also caused more BS to fly than you can shake a stick at.

Many gunsmiths are extremely good at the traditional "smithing" but have no knowlege of the science of heat treating and it's testing. They draw a file over a ring and immediately pronounce it's suitability of use. This is pure BS in it's totallity and to be cautioned against. Many experiences with Eddystone receivers are the culprit of this as they are often so hard as to not allow a sharp file to even bite a little bit. Thay can be "glass hard"

A PROPERLY HARDENED RECEIVER CAN BE EASILY FILED OR MACHINED WITH HIGH SPEED STEEL TOOLS

36-40 Rc is not so hard as to be easily detectible to a novice.

Here's one serious drawback to our terminology. The hardness is usually spoken in terms of Rc or rockwell "C" meaning read on the "C" scale. Further it's the most understood hardness scle a going. Most folks can relate to this as it's what's used the most.

Here's the wrinkle.....you can't inspect case hardening with a tester that tests on the "C" scale. That tester applies a cone under much greater force than the case depth supports and the cone actually can break thru and provide a reading that is far too soft.

The correct machine to use in the case hardening metals is called a superficial hardness tester and inspects to 15, 30, or 45 KG loads. This is referred to as the 15 N, 30 N and 45 N scales and there are charts that show the corresponding hardness in the "C" scale.

Who knows how many perfectly fine actions have been heat treated or discarded because the inspection procedure was just plain wrong!!!!! The action was fine and inspected incorrectly and pronounced soft!!!

I've even seen this in industry by folks that don't understand this. Parts rejected because of improper testing.....what a waste.

I've even seen design engineers specify case hardening to the Rc scale to further complicate this matter.....

The bottom line is that there's only one way to properly check a Mil-surp Mauser for hardness and that's with a superficial hardness tester (not a typical Rc tester) on (hopefully) the 15 N scale. Any other method must be regarded as "suspicious" and ignored. Any company in the case hardening business has these machines. They are rarely found even in other industrial settings.

I don't know that 15N is the ONLY way, 1KG Vickers and Knoop readings have there place also

Not a hardness expert, I used to do calibrations of testing machines however.

you may be right..... I'm not familiar with the european styles...is this British I assume???

They're the 2 scales used with a Micro-Hardness tester.
Vickers started in England, Knoop is US (a diamond is defined as 1000 Knoop in hardness). Vickers uses a square, diamond, indenter and Knoop uses a long narrow, diamond, indenter.
You measure the length of the diagonal with a scaled microscope (both and average with the Vickers, just the long diagonal with Knoop) to get hardness, similar to how you measure a Brinell dent to get BHN.