Uncharted: Drake's Fortune

This trophy will pop after successfully killing your 50th enemy with the PM – 9mm. This gun features in every chapter of the game, you will earn this trophy indefinitely throughout your two mandatory playthroughs.

Weapon Kills Trophies workaround and cheats:
After earning some of the trophies you will also earn some points. A certain amount of points will allow you to use the game's cheats, which can be accessed on the pause menu by going into the 'Bonus' menu.

After the completing the game under all difficulties and a few or if not all Misc Trophies, you are now at liberty to switch on the game's cheats whilst playing by going: => Bonuses => Rewards.

Below is the list of cheats which you can enable at anytime during gameplay, providing you have the points to do so.

PM - 9mm Semi-automatic Pistol – 300 pts
AK-47 Assault Rifle – 300 pts
Moss - 12 Shotgun – 300 pts
92FS - 9mm Semi-automatic Pistol – 500 pts
Wes - 44 Revolver – 500 pts
SAS - 12 Shotgun – 500 pts
Mk-NDI Grenades – 500 pts
Desert - 5 – 700 pts
Micro - 9mm Fully Automatic Pistol – 700 pts
MP40 Submachine Gun – 700 pts
M4 Assault Rifle – 700 pts
M79 Grenade Launcher – 900 pts
Dragon Sniper Rifle – 900 pts

Infinite Ammo – 860 pts

You need a total of 8,360 pts to buy all of the above.

Before you attempt to clean up the Weapon Kill Trophies. For the workaround to work you really should have completed the following:

Collected all of the Treasures – 175 pts
Completed all Melee Trophies – 160 pts
Performed all Headshots – 65 pts
Run and Gunner and Hangman – 20 pts
Completed the Game under all Difficulties – 350
Earned Survivor Trophy and Expert Dyno-Might – 80 pts

This all totals 850 pts.

Throughout your first 2 playthroughs you will have more than likely earned a few of the Weapon Kill Trophies anyway so you should have over 900 pts, which is the required rate in order to unlock all weapons and to be able to use them at your leisure. Once you have the points, go to Chapter Select and play Chapter 17, You will have lots of unarmed enemies which you can run around killing liberally. Whilst in play press => Bonuses => Rewards. From here you can toggle what weapons you want, obviously choose the ones that you do not have the required kills for its respected trophy.

Note: Make sure you are not aiming, hugging a wall or hanging from a ledge otherwise you will not be able to activate the cheats. The reason for this is because they become disabled if you do any those 3 motions. Once you have selected the weapon, your next move is to go round killing. The enemies keep on respawning, just do not climb the rope which Elena swings over towards you. Once you have the required amount of kills for the weapon, simply turn on another weapon cheat which you need the kills for. Keep doing this until you have all of the Weapon related Trophies Unlocked.