30 Kills: Mag 5 Trophy in Uncharted 3: Drake's Deception

The Mag 5 is a powerful Handgun in the same category as the Wes 44, a high calibre one shot pistol but with a low ammo capacity. It can take out even armoured enemies with a headshot so is a pistol worth saving for very rough areas. You first encounter this gun in Chapter 8 - Citadel, inside the tower as you try to ascend to the roof. Here's a couple of good farming locations:

Chapter 8 - Citadel

As you pick it up for the first time you may have a few stragglers to take out, simply leave one man standing in this area and one shot kill the rest of the guys, restarting the checkpoint as required.

Another and probably better spot is further into the same chapter, right after you and Cutter get separated from Chloe and Sully following an onslaught of RPGs. When you regain control you will be encouraged to take the quiet route. When you follow Cutter into a tunnel structure you will see the first guy walk away, follow him and stealth kill him to take his Mag 5. Now just take everyone else out with the ammo, restart the checkpoint as required.