The U.S. Marine Corps is going to switch to hollow point rifle and pistol ammunition. Actually SOCOM (Special Operations Command) and the Marines have been using hollow tip bullets for over a decade, mostly in Afghanistan and, for SOCOM, worldwide. This use of hollow point, rather than full metal jacket ammo, began with pistols. SOCOM experimented and found that a full-metal jacket 9mm round has a one-shot stop rate (OSSR) of 70 percent, while the best 115-grain 9mm hollow point has a one-shot stop rate of 91 percent. OSSR is largely just a way to measure the relative stopping power of different (in terms of caliber, design and weight) bullets and forcing the target to drop because of death or incapacitation. Much depends on where you shoot someone and SOCOM found that the highest OSSR was achieved with a .45 caliber (11.4mm) pistol firing hollow point bullets. That gives you something like .99 OSSR and the OSSR stays high no matter where you hit someone. Basic OSSR is calculated assuming a hit in the center of mass. It was long known that hollow point does more damage but lots of battlefield experience since September 11, 2001 made it clear that hollow point ammo was a key tool in limiting the effectiveness of Islamic terrorists.
There is a popular and long-standing myth that hollow point bullets (that expand on hitting and create larger and more damaging wounds) are illegal according to the Geneva Convention. That treaty does not mention bullets. The later 1899 Hague Convention does and it prohibits some types of hollow point bullets. The U.S. never signed the Hague Convention and was never bound by it. But in deference to allies who did sign American forces have rarely used hollow point bullets. One frequent exception was for certain types of commando operations, like hostage rescue. No allies ever complained about this.
Shortly after 2001 SOCOM and the marines, responding to complaints from troops that the standard 5.56 and 9mm full metal jacket bullets were not doing enough damage to stop fanatical Taliban fighters, began issuing hollow point bullets and troops were satisfied with the improved stopping power. As a result the Marines are going to switch over to the MK318 hollow point (or “open tip”) round for its assault rifles and machine-guns. The existing M855 full metal jacket rounds will be used up in training exercises. SOCOM has also switched to hollow point for pistols (9mm and 11.4mm) and rifles. The army is watching all this carefully.