The account of the unfortunate accident with the 6mm Lee Navy was very informative. The two important clues were the pierced cartridge case and the fact that the bullet was still in the barrel.
I am confident that what we have here is a case of excessive case mouth thickness. In other words, when the powder charge ignited, there was insufficient clearance between the case neck and the chamber walls to allow the bullet to exit the case. This is a condition which results from the wrong ammunition being used in a rifle (8mm Mauser in .30-'06, a frequent occurance after World War I, causing numerous blowups in Model 95 Winchesters and carbon steel [low number] Springfields), from excessive case length (the case protrudes into the throat of the chamber) or, as probably was the case here, from cases formed from another caliber cartridge case, with no attention paid to neck clearance.
Since the 6mm Lee Navy cartridge has been obsolete since the '30's, the unfortunate handloader probably formed his cases from something else. .220 Swift cases are the obvious choice, since the Swift was based on the Lee Navy case. Somewhere in the course of forming, the critical neck dimension was overlooked, which caused the catestrophic rise in pressure, the ruptured cartridge case and the destroyed action.
The sequence of events was apparently this: upon ignition, when the pressure began to climb inside the cartridge case, the bullet, which would have normally been propelled down the barrel, increasing the volume of space into which the burning gasses could expand, remained obstinately stuck in the neck of the cartridge, causing the gas pressure to continue to rise to the point where the cartridge case, inevitably the weakest link in the system, gave way, releasing gas under pressure in excess of 50,000 lbs/in/sq
into the action, wrecking it, and propelling the fatal fragment into the shooter's brain.
A practical test for insufficient neck clearance consists in attempting to insert a bullet into the neck of a fired, but unresized cartridge case. The bullet should fit closely, but not encounter resistance when pushed all the way into the fired case. Had this test been performed with the fired case found with the loaded ammunition, the final bit of evidence might have been in place.