The M79 isn't the most common of guns but when you find it you can kill multiple enemies with one round. The M79 is first found in chapter 19, whenever you find it make sure to pick it up and try to make the ammo count. It will be one hit kill on Normal and below but it will take a few on the higher difficulties. The M79 is a grenade launcher and is NOT to be confused with the GP32 - BND. If you don't have this after your two playthroughs you can grind it in the following chapter:
Chapter 26 - At the very start of the chapter you are on a ledge with Sully. You automatically get an M79. Firstly shoot the guy who is on a tower, make sure you kill him. This will cause all the enemies to go alert. Look in the distance at the gap between two towers, a few enemies will be shooting you from here. A well placed grenade will kill then all. When you run out of ammo just kill yourself or do the whit to main menu method. Trophy will unlock after getting 50 kills with it.