Perform a kill with every firearm (1)

Every time you get a new weapon in a chapter make it a priority to kill an enemy with it. When you kill enemies look for the weapon they drop as well as it may be unique. You will not get this trophy until very late in the game because the last weapon (Alien one) is not available until one of the final chapters. You may only need to kill enemies with guns and not explosives, I killed enemies with everything though.

Full Weapons List:
Pistol (Starting Weapon)
Scar (Starting weapon)
FY71 (Default enemy weapon)
SMG (Enemy weapon, found in chapter 4+)
Precision Rifle (Sniper, picked up in various levels)
Shotgun (Common enemy weapon)
Grenade (Found in various chapters)
C4 (Found in chapters where you have to take out objectives like tanks, AA guns, etc)
Missle Launcher (Found in various chapters where you have to take out objectives)
Guass Sniper (Given to you as a pick up at the end of Onslaught, get from VTOL)
Minigun (Picked up in later chapters, some enemy nanosuits carry it, for example in Chapter 5)
Alien MOAD (Prophet will drop the weapon shortly after you meet him, pick it up)