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I don't understand why one round of a 250 grain bullet going 973 fps would blow the cylinder. The pressures should not be that high. Unless, this is a example of metal fatigue.
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Think of it this way. If all that pressure escapes in all directions when the chamber comes apart, but there is a still enough of it directed through the barrel to move it at 973 fps, imagine how fast that bullet would be traveling if the chamber had stayed together and directed 100% of it through the barrel like its supposed to.
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If the OP had not put in the bullet velocity, I would be in the camp that this was a propellant overcharge. I would say 90 + % percent of blowups is due to an overcharge of powder, but here, the velocity is low.
If the pressure was sufficient to blow the cylinder, that same pressure is available to move the bullet. We know pressures were high enough to blow the cylinder, but were they overpressure? I don't know. Assumptions that a burst cylinder will reduce bullet velocity is just as untested as my assumption that it won't. Pressure drop in revolvers is quick as pressure vents out in the cylinder gap, I have seen Speer data on this. I doubt there are many blowup tests where the velocity of the bullet was measured at the time of blowup.
What we don't know is how many over pressure rounds were fired through this gun before the cylinder ruptured. Since the OP has not replied, I don't know how many rounds have been fired through this revolver and whether they were overpressure or not. But, given enough overpressure rounds, cylinders will fail through metal fatigue and they will rupture right through that cylinder notch. The cylinder notch is the weak point in cylinders.