I doubt that the hull has sufficient strength in the walls to gen up enough force to hold the shot long enough to cause much, if any, increase in pressure. The hull should fracture at something like 650-700 psi chamber pressure. This would be quite early in the burn. I'll post calculations if anyone is interested.
The pattern might be a different story. If the hull ruptured below the shot column, you might have a "sortaslug." If the hull ruptured at the crimp base, I'd expect it to act like an overshot wad and spin off shortly after muzzle exit with minimal effect on the pattern.
Back in the bad olé days, I loaded AA's until they fell apart. I never noticed any deterioration in clay breaking between new and rag. But, I've never run pressure tests, either.
DDA
