Both bombs contained a mechanical time fuze that could be set to open the cluster at a preselected time between 5 and 92 seconds by triggering a burster charge. The case sides and ends sprung open, allowing the bomblets to fall out. The case sides were cup-shaped to retard the fall of the bomb. The manual warns that "Use of the M131 fuze is not recommended in areas that are expected to be occupied by friendly forces as they constitute a potential booby trap".