Cook off has always been a major problem with caseless ammo.
It turns out that heat has been the enemy of the caseless cartridge concept. That “old-fashioned” brass shell does more than just hold the gunpowder, it also serves as a heat sink to absorb and remove most of the heat generated by the burning powder. Without that brass shell, all of the heat goes into the chamber. Pretty soon it’s red hot, hot enough to ignite a caseless cartridge on its own.