The M24 is a Sniper Rifle, while the IA2 AR is an Assault Rifle. This is easily done in Deathmatch and can be unlocked on any map, wether it be a DLC map or original map.
Make two different loadouts, one with the Sniper Rifle, and the other with the Assault Rifle. When the game starts, aim to get about five to six kills with the Sniper Rifle, in hopes that each of the kills are on a different enemy player. Switch to your Assault Rifle class and get another five to six kills and hopefully, one of the enemies you kill will have fallen victim to both your weapons. The achievement will unlock once the same enemy player is killed with both the new weapons.