Grenades can traditionally be grouped into two categories: offensive and defensive. Defensive grenades are intended for use in circumstances where friendly and enemy positions are a known element, allowing for a greater degree of lethality (usually accomplished through shrapnel). Offensive grenades, on the other hand, are meant for use in the messy quagmire that is many modern battlefields, where a cloud of deadly shrapnel could feasibly kill or injure friendly forces nearby. These grenades tend to be less lethal, relying on a powerful explosive to kill or stun enemies in the immediate vicinity without causing much harm to nearby friendly forces.

Currently, the Marine Corps relies only on defensive, highly lethal grenades like the M67, which has a steel body that breaks up into small, hot, fast moving shrapnel that can kill any opponent within a 5 meter radius or so, and maim anyone within 15. This grenade, while highly effective in open spaces, can also result in nearby Marines getting hit with shrapnel in close-range urban fighting, and with the U.S. military transitioning its posture toward the possibility of future wars against near-peer opponents like the heavily industrialized China, the Marine Corps is now considering the purchase of a series of stackable offensive grenades that would allow Marines to choose how much firepower they want in their explosives right in the thick of the fight.

(Warning: Graphic Language in video)

“If you throw an M67 into a room it doesn’t discriminate,” explained Pat Woellhof, the director of Marine Corps operations for Nammo.

Enter Nammo’s Mark 21 grenade, which was “specifically designed for use in closed rooms, such as bunkers, buildings or semi-closed areas like trenches,” according to its creator. These cylindrical grenades can be used individually or stacked in groups of two or three. Each grenade has its own internal fuse, but when stacked on top of one another, the fuses link to provide a single, simultaneous explosion. Because these grenades are intended for use in close quarters with other friendly forces nearby, they were designed not to produce as much fast-moving shrapnel as alternatives like the M-67, making them safer to use inside buildings and in alleys.

Nammo’s series of grenades, with the stacked Mark 21 grenades shown on the left. (Nammo)

But don’t let the lack of shrapnel fool you. With 130 grains worth of explosive material in each Mark 21 (or about 50 less than you’d find in an M67), they may be weaker per grenade than the Marine Corps‘ current stock, but a three-stack would provide 390 grains worth of destructive power — enough, Nammo claims, to take down a small building.

The modular design is a new approach to the time honored grenade but seems feasible that it could offer ground troops a safer alternative to the one-size-fits-all M67. According to Nammo’s assessments, one Mark 21 could kill or stun opponents in a room, two would kill everyone inside the room, and three might just take the building down on top of them.