Fundamental to this discussion is a basic understanding of triggering mechanisms for those ordnance types most likely to become impaled in the body. These include mortars, Rocket Propelled Grenades (RPG) and 40 mm projectiles (rifle-launched grenades). A basic, unclassified understanding on how triggering and arming of the explosive round occurs will ensure the surgeon and surgical team avoid causing the ordnance to explode.

Previous reports of UXO reveal that the impaled round was some sort of propelled explosive device. These rounds basically consist of a propulsion system, a trigger mechanism, and a main explosive charge.

It should be assumed that a malfunction occurred causing the ordnance not to explode on impact into the patient. One should always assume that an inadvertent bypass of the safety mechanism or a malfunction could cause the ordnance to explode. All retained ordnance should therefore be considered “armed” or activated to a degree that final triggering of the fuse would cause the ordnance to explode. Fusing and triggering mechanisms vary by the type and variety of ordnance and may even vary within the same type of ordnance depending on where the ordnance was manufactured.

The trigger is generally located at the tip of the main explosive charge. This trigger activates a firing mechanism that impacts onto a percussion cap that activates a detonator. The detonator explodes, thereby igniting the primary explosive charge.

There are a variety of arming mechanisms for explosive ordnance. Typically, a mortar or rifle grenades (e.g., 40mm, M203) becomes armed and capable of exploding based on the number of rotations or spins completed after leaving the launching tube. Upon impact, a nail-like device located in the cone or nose of the device is pushed down into a fissile explosive that detonates the actual explosive charge. Pressure on the nose of the mortar may also trigger the device to explode.

Piezoelectric crystals are also utilized as another arming mechanism. This is most commonly employed in an RPG. The rocket consists of a propulsion device and stabilizing fins for flight. The trigger utilizes a piezoelectric crystal at the tip of the projectile that generates an electric charge upon direct impact with the target. This charge ignites the detonator that explodes the primary charge. This crystal may also be activated upon exposure to light, electricity, or thermal energy. Therefore, the explosive device may be triggered by reorienting the patient, shining direct sunlight on the crystal, or providing a direct electric current to the device such as contact with an electro cautery device.