With regards to Andy from Ranger Point. Surface hardening, through hardening, and precipitating hardening a combination thereof are four different animals of metal characteristics. A comprehensive metallurgical study would have to be undertaken to determine the evolution (devolution) of Marlin's applications. You see the results and failures every day in your facility and it does not take a rocket scientist to determine that parts are failing in many of the new guns. It would be nice to know the definitive reason, but we may never know, Just by changing one little spec of the same garde of material can make a huge difference. An example is in the 8620 series. E8620 is electrical furnace grade, and E8620M, a newer kid on the block have their subtle differences.
Forging parts imparts totally different mechanical characteristics than investment casting, cold rolled or hot rolled material. I suspect the new Marlin, perhaps in a carry over of the failing Marlin may have made changes. You can case harden the lowest grade of steel so the surface is harder than a file, but under that case (.003-.018") thickness the material is still cheese. Picture an egg. I'm seeing historically that the 41XXX series of chrome moly and the 86XXX series of nickel toughened steels seem to work best in receivers, (barrel extensions) bolts and high stress parts. Most M16 barrel extensions are case hardened 8620 steel - a very common metal used in the gears of transmissions and differentials. An application that requires toughness, impact resistance, and anti wear that a cases hardened surface produces as long as the loads do not exceed the underlying material strength.
Using the wrong Rockwell scale on the wrong material will result in erroneous readings. Case hardened steel, for instance, requires a lighter load with a small diameter ball on the 'N' scale in order to not pierce the 'case depth'. The resultant reading is then converted to the 'C' scale to determine how hard the thin 'cased' surface is.
There are a myriad of other processes that if omitted can lead to premature failure. Tempering, stress relieving, shot peening, and cryogenic treatments add to the production costs, but if omitted will cause problems downstream. Overusing processes where not needed just adds cost making your product less competitive price wise. The savvy manufacturer will find that delicate balance to produce the most effective parts at the most effective cost.
I fear that many of these corporate takeovers do not fully appreciate what it takes to build a sound product, at a fair price, which results in a loyal customer base.
Companies with lacking ethical compass bearings or suffer from incompetence, ignorance, or even worse, disregard, will eventually fail in the market.
AC