Quote Originally Posted by tiriaq View Post
FWIW, I have salvaged some really horrible, rusted No. 4 specimens. I have soaked the receivers in industrial rust remover. Interestingly, the hardened & non-hardened areas show slightly different colours when removed from the bath. Based on these observatons, I would suggest that receivers were induction hardened in the camming and locking areas, but not end to end, through and through. I would not expect to see cracked Lee Enfield receivers, because they are not hard enough to be brittle.
If you can find a WW2 Lithgowicon, with unworn original finish, you will be able to see the discolouration from selective area hardening in the same regions.
The junker Lithgow I have here showed a very noticable difference in appearance when cold blue was applied.
A previous refinishing had deeply scored the surface of the receiver and left the hardened locking surfaces standing proud of the surface, with shreds of brass wire stuck in the surface around them.
I had thought at the time that it was a reactivated drill rifle, probably is anyway, that had the cut away locking surfaces rebuilt by welding, the difference in the surface was that distinct.

I have a low numbered '03 receiver which cracked through the right siderail when tapped with a light hammer. These receivers were through hardened and drawn, and it is well established that some are brittle.
The theory was sound but the execution was poor. They weren't meant to be hardened all the way through, and only a small percentage were.
Poor quality control and out of date methods doomed those receivers.

Mauser receivers tended to be made from clean low carbon steel and carburized. This is why badly worn, or over loaded mauser receivers can show setback in the locking area. I suspect that the left sidewall thumb notch cracks occur because the section is thinner there, and consequently there is less core relative to the thickness of the case. Leaves the receiver brittle in that area.
The use of WW2 surplus Germanicon ammo of unknown qualities is probably why so many Mausers have set back, and why some developed cracks.
Some early WW2 production AP MG ammo has a far higher working pressure that can stress even LMG actions. They quit making that ammo in 1942 due to shortages in tungsten, but it shows up every so often.
Ammo interchangability between rifles and LMG often goes only one way. The LMG can safely handle and operate with infantry rifle ammo, but MG ammo is often loaded to pressures that can damage rifles.
The MkVIIIZ MG ammo was not considered suitable for the SMLE rifles, and was authorized for use in the No.4 only if MkVII ammo was in short supply.
There are references to MkVIIIZ ammo that was far beyond acceptable tolerances, and showed flattened primers, "like the primer was painted on" as one officer put it. Ammo of that sort could damage any rifle.