Final answer:
Spontaneous combustion occurs when the heat of the barrel causes the propellant powder to ignite without the primer being struck. This can happen due to high temperatures or a spark from friction.
Step-by-step explanation:
When the heat of the barrel is high enough to cause the propellant powder inside the round to ignite even though the primer has not been struck, it is known as a spontaneous combustion. Spontaneous combustion occurs when a substance oxidizes and releases heat without an external ignition source. In the context of firearms, this can happen due to high temperatures or a spark from friction.