Where is the rest of the barrel? was it cut away from the receiver, or was it blown completely off?[

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Unless a faster burning powder was used than what should have been used, it seems unlikely that a double charge or overcharge would have done this, I usually opt for a powder that nearly fills a case imho.
Were the bullets seated too deep?
was the third shot a squib- leaving the bullet partway down the barrel? - This sounds like the most likely scenario, as it can be easier to accidently unot charge a case with powder than overcharge- especially if the loader is putting uncharged cases in a loading block.
I' M not sure what rifles are proof tested at, but they should hold up to 100,000 psi, I' ve read about how the military tested ' 03 springfields which would start to fail at 125,000 psi and this was 60 years ago probably using inferior metallurgy than what is used today. Either way, that is a heck of alot of pressure- about twice what the typical magnum load produces.
Handloading is serious business. It requires the kind of alertness and awareness of danger as operating heavy equipment- only difference is that a small mistake won' t hurt you or someone else now, but days, months, or years later.