i've heard all kinds of things over the years...8620...1018...h.r.s. ...4140.

seems the itm/oow is cased, as is the krinks receiver (according to their advertising).

just remember, low to mid 30's is fine for hardness. for good tool steels that can be reduced to the mid 20's (rockwell 'c' scale). the receiver need not be as hard as glass to have a good service life. it only has to be hard enough to prevent premature wear from rubbing (carrier), pivot hole stretching (hammer, especially) and rivet hole working (very unlikely, but i have seen it occur).

too hard and you really have to start concerning yourself with cracking of the upper rails.

talk to denny at impact guns.

iirc, i think he said it cost about a c-note (which is a deal. the lab i deal with charges us 2 bones per analysis).

the physical and chemical certs are traceable to n.i.s.t. and are very accurate.