As someone who has been making custom knives for the last 10 years, I think a lot of ya'll are a bit confused by this thread. There's been some correct things said about Rockwell, and a whole bunch of pure BS, far as I can tell. This is what I KNOW...not my opinion.
1. As stated the Rockwell C scale is for martensetic(heat treatable) steel. It is a measure of hardness only, however that number can be of considerable significance as far as receiver strength goes. What the Rockwell number says is that the steel has been HT'd to it's optimum value and that the steel will perform as expected. I can take several different knife steels..440C, ATS-34, O-1, D-2 etc and heat them to any flippin Rockwell number I chose. At that number some will be at what is known as optimum Rockwell, the others will either be too soft or too hard and brittle. Where the Rockwell C number is important in receivers is that over heating the receiver to too high a number will cause the receiver to become brittle. A brittle receiver will crack. That was a big thing at one time with the low number/high number Springfield rifle receivers...some were inproperly heat treated at the armory and were too hard and brittle to stand constant 30-06 pressures without cracking....they had lost their ductile strength. If you're thinking that this indicates that the rockwell C number means whether a receiver is strong or not....technically you're wrong, since it's (again) looking at hardness. However to understand the rockwell number's signifigance, you have to know what the steel is and what it's proper rockwell should be.
To re-state, all martensetic steels have an "optimum" hardness, and a specific steel is chosen for a job based on it's fitting those characteristics when properly heat treated. Should a rockwell test indicate improper numbers for that particular steel, then it will either be too soft or too hard/brittle...either one of which can cause problemsJust about all modern rifle receivers are between 25-30 at the Rockwell C scale. For the practical, everyday user of such products then, the number does indicate strength, however technically incorrect that is to the metallurgist.
2. Based on what the factory said years ago, and what was posted at the Yahoo NEF forum back then, the earlier receivers thru the SB1 are made of "sinctered iron", not alloy steel. In order for a iron based metal to be considered steel vs iron, the amount of carbon in the matrix has to be at least .4% by volume...if it aint, it ain't steel.
3. Non-martensetic "steels" get their strength by being work hardened, rather than heat treated. Soooo, forging a scintered iron receiver gives it its strength. Doing a rockwell test on it using the C scale gives erroneous results... I think the proper scale is the B scale for those types. The SB1 receivers are not stronger, they are being subjected to the wrong test/rockwell scale.
4. Were you to cross section an older receiver and the newer SB2's, you wouldn't have to be a metallurgist to visually understand why the older receivers are weaker. Scintered iron has very large, irregular grains, arranged in a random manner with lots of air space between the grains. A good alloy steel will have smaller grains, more regularly distributed with much less space between the grains. Try considering a stone fence made of large irregular rocks and one made of small well fitted and shaped pebbles. At the same size and thickness, the pebble fence will actually be much stronger.
5. While the Brinell numbers give you a much better indicator of load bearing strength, they are still not quite accurate, but much closer than any number the Rockwell will give you. Gun steels also need special characteristics such as being ductile(flexible) to accommodate the large pressures in modern cartridges and to allow passage of a tight fitting object such as a bullet down a tube that is the same size, usually, as the bullet. Try pounding a 1/4 inch hard steel rod into a 1/4 inch hard steel plate, and you'll get my drift.
6. The older receivers could handle the fairly high pressure 22 Hornet at it's nominal 40K CUP, since there was enough sheer mass to offset the rather small surface area of the rear of the Hornet case. Clumsy, but it works as long as you don't change any of the parameters.
