Manstopper
Can refer to any large caliber gun or bullet that will immediately "stop" a person with one hit. You might live from receiving a limb shot. But there is little to no chance of surviving a hit to the head or body. .50 AE, .45 ACP and .357 Magnum are popular calibers know as manstoppers. The larger the better.
Dirty Harry didn't mess around. His .44 Magnum was an absolute manstopper. Do you feel lucky punk?