Lots of clues here, but not nearly enough information to really make any judgement. Having the receiver grenade to pieces suggests improper heat treatment, but there are merely vague clues as to what caused it to fragment. A complete head separation is more likely to be a sign of excessive headspace than high pressure. That can be a defect in the gun, or from using brass that is somewhat brittle and has had the shoulder set back too much during resizing. I had the latter happen with a VZ-24 Mauser rebarreled to .22-250 when I foolishly used some old brass a buddy gave me. Primers of the two cases fired prior to the head separation were flattened severely even though the load wasn't hot. But it was a lot warmer than this 5744 cast bullet load rated at 30,00 psi. Yet there was no damage to my gun. Plenty of gas escaped, and I was totally blinded for about 20-30 minutes and had numerous brass and powder particles hit my face. After I began seeing slight blips of light, it took about another 30 minutes for my vision to clear up. Young and dumb, I had no shooting glasses on at the time. Later firing with brand new brass that was properly resized for my chamber was no problem at all.
During a head separation like that, the case head is unsupported by the bolt face because the firing pin pushes it forward in the chamber perhaps several thousandths of an inch until the shoulder stops it. When the primer detonates and ignites the powder, the case expands and seizes the walls of the chamber. The unsupported head slams back into the bolt face, and this is what flattens the primer. Normally, with fresh brass, the case stretches and there is no problem at the time, but it might partly or completely separate after more reloading when the shoulder is again set too far back during resizing.
So was that sudden impact from the case head slamming the bolt face enough thrust to shatter the action on this low number Springfield? Or did the threaded portion of the barrel expand slightly due to excessive pressure... even though it appears un-deformed and not bulged. You would need before and after measurements to know for certain. If there was some slight expansion, then I would suspect either some detonation phenomena due to excessive air space... or an accidental double charge.
I'm wondering what kind of pressures the 44 gr. of 4895 charge under a 150 gr bullet that Michael used in this same rifle developed? Certainly more than the 30,000 psi from the Accurate 5744 load, That is pretty mild to do the type of damage described. This is why I hate even using any loads that will not overfill the case with an accidental double charge. But of course, I have done it thousands of times, especially with handgun reloads. Any of us can get distracted and make a mistake. That leads to the question of what pressure a double charge of this 5744 would develop. Would it actually be enough to rupture the chamber, or more likely, just enough to slightly expand it and shatter a somewhat brittle receiver? I would want to carefully measure the barrel thread diameter, and pour a Cerrosafe cast to verify that chamber dimensions have not expanded slightly. Maybe insert one of the previous fired cases all the way and check for any slight wiggle or slop. Even with a double charge, I would not expect any dimensional difference in the chamber area that you could see without measuring. But the grenade-like damage described here doesn't inspire me to want to shoot low number Springfields. I'm thinking of P.O. Ackley's action strength tests where he progressively loaded hotter and hotter loads in numerous rifles all chambered in .30-06. The action from a 6.5 m/m Type 38 Jap Arisaka was the last man standing, and finally, he filled a case with some pistol powder and weakened the case head by filing a notch in it. When he fired it with a long string on the trigger, the barrel threads stripped and sent the barrel down range. But the Arisaka action held.
