ok, here's my stab at it.
Firstly, there is brass flowed into the extractor cut and heavy slag deposits there, showing this area saw flame burning before the receiver let go. This tells us the failure originated in the chamber while the bolt was locked up. It would not be a headspace failure. Headspace failures will typically result in brass being contained on the extractor side and you have a circular base failure that blows out the gas relief hole on the left side of the receiver ring. These failures, typically, only occur if you are using brass with insipient separation already started, typically other than some gas in the face and a re-cocked bolt, you have few other symptoms of a case head separation. I've never seen one grenade an Enfield if the charge was otherwise right.
Second, the barrel looks to be a milsurp barrel. A military chamber would not exhibit neck tension issues, so I would rule that out unless you were shooting projectiles measuring at least .314" in diameter. There are pipe wrench marks, meaning at some time someone tried to remove or install the barrel with improper tools. Possibly they used too much force putting the barrel on and generated a hairline crack in the receiver ring, but without being to NDT your receiver fragments and run an LPI/dye penetrant test on the remaining breech face, I would be hesitant to blame that.
I know you think you did not overcharge that case, but frankly, the failure mode you exhibit is characteristic of a reloading error. Is it possible you recently reloaded a faster powder in your setup and didn't get al the old powder out? Maybe this was the first round you loaded? Did you buy your 4895 new, or used like at a gunshow?
The failure you depict shows the rear of the case expanded too rapidly and ballooned into the extractor cut void. At some point before the projectile could clear the barrel, the brass failed near the rim and dumped expanding gases into the receiver ring just aft of the barrel while the bolt was locked up. This would only really ever happen with a fast powder, probably a pistol powder. When this happens, the small gas vent hole is not sufficient to drop the pressure curve and the top of the receiver ring lets go. Crack propagation will typically start at the receiver C-cut (clearance for loaded round ejection) and propagate forward to the nearest receiver facing flaw, or a micro-tension crack from barrel install. Then it totally lets go and you get what we are seeing.
The steel in your failure area looks like typical failure for surface-hardened low carbon steel, the structure looks smewhat normalized (i.e. a pretty even mix of pearlite, austenite and martensite), a good thing, and not as coarse and angular as an embrittle martensitic stucture failure. From what little I can see in your photos, this tells me the receiver was likely sufficiently ductile at the core, despite likely being surface hardened in a carbon or bone-charcoal mix at manufacture. This could be verified through charpy testing a sample. The bulged appearance supports the tough-core theory with a relatively high modulous of elasticity.
Translated to English, it means I think your receiver was manufactured properly but you asked it to do more than it could through reloading error. If I had to bet the farm, I'd guess you had some pistol powder in your casing, not just H4895.
...wow, size of a bull huh?
