Massive chamber pressures with small charges seems counter intuititve. What's not appearant is that powders, especially slow powders, don't burn at a linear rate. The burn rate changes dramatically and quickly as pressure increases. In extreme instances, increased resistance to bullet movement can stop it long enough for pressues to skyrocket in a thousanth of a second.
The process is thought to be the bullet moves forward under the pressure of the primer but the light powder burn intially has too little pressure to continue to move it. If the bullet stops, or even slows very much, the remaining powder will suddenly burn MUCH faster than normal. Pressure can increase so rapidly the intertia and resistance of the lodged bullet can't move and expand the chamber volume fast enough, so things start to seperate.
It's rare in handguns EXCEPT with certain powders such a H-110, W-296.
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