Mercy

 is a unique 40mm grenade machinegun in Fallout: New Vegas.

Characteristics
Its higher damage comes at the cost of using 40mm grenades, which are less common and more expensive than 25mm grenades. It cannot be modded, and fires much more slowly than a modded grenade machine gun. The larger size of the 40mm grenades also restricts its ammo capacity, which is 12 lower than the regular machinegun.

Variants

 * Grenade machinegun

Location

 * Dead Wind Cavern - By a dead Brotherhood of Steel soldier along with 40 mm incendiary grenades, 40 mm grenades, brotherhood power armor, and brotherhood helmet.It is guarded by many Young Deathclaws, Deathclaws, Mother Deathclaw and the Legendary Deathclaw.

Behind the scenes

 * The weapon is itself a reference to the Fallout 3 weapon Vengeance. Like Mercy, Vengeance is hidden in a cave with a high concentration of Deathclaws (the Deathclaw Sanctuary). There is also the obvious connection in the name of the weapons (Mercy vs. Vengeance).
 * The words "HEI GUI BYE-BYE" on the left side of the gun suggest that this gun may have been used to hunt the Chinese in the Hei Gui stealth suits during the war. This might explain the large amount of rust from use or age.