Mowing them down

This achievement requires the player to reach 500 total kills using a gatling gun. Since these are only featured in missions, the best way to obtain this wud be to replay the specific missions from the stats option in the pause menu. Missions to feature Gatling guns include "The Assault on Fort Mercer" (however it is not used much compared with other missions in which a Gatling gun is used) "The Great Mexican Train Robbery" and "Mexican Caesar" (this probably being the best as the majority of the mission is on a gatling gun on a train).