Another factor is that if the slide starts to move before the pressure has gone down a little, then it will rip the head off the case.
There is an incredible amount of presure on the walls of the case gripping the chamber, while the cse wall is fairly thin.
Notice in rifles with excessive headspace the case stretches, as the head is blown back to the breech but the case walls stay where they are until the pressure subsides.
Eventually the cases thin near the head and break off if they have been reloaded too many times.
All firearms do it to some degree, but hotter loads or greater headspace accellerate it.
If a semi blew open at high pressure you would rip the head straight off and let a lot of hot gases come straight out the back at high speed, with potential for damage and injury.
Just as a quick example, using a 9mm cartridge,
the area of the cartridge wall is 537mm squared.
Assuming a coefficient of friction of 0.3 ( cartridge brass on a smooth chamber) and a peak pressure of 35000 psi or 241 MPa;
the gripping force will be the pressure x coefficient of friction multiplied by the area, giving a figure of 38,873 Newtons of force.
The breaking stress of brass is 550MPa.
Assuming a wall thickness of 1mm, the sectional area will be 25.13mm squared. ( I think the wall thickness is smaller, but have assumed 1mm to be on the safe side)
Therefore the stress on the cartridge wall iff the head is unsupported will be 1,546MPa before the case wall will pull out of the chamber ( stress being force/area)
This is 3 times the breaking strain of the brass, so the head will come of before the case will move at peak pressure.
The force on the head at that pressure is the pressure x the case head area, which comes to 15350N.
The stress on the case wall will be this force divided by the case wals section area, giving a stress of 610MPa, so with a rupture pressure of 550MPa, there will be enough force on the case head to break it off.
However, if you delay it just long enough for this peak pressure to decay a little then it will not rip the head off.
Sorry for the quick and dirty calculation, and not showing all of my working out, but it would have been too messy to type it all out.