The problem I found with the Hunting Rifle is that they are not as common as other guns, and when you do find one, you don't find much ammunition to make this achievement any easier. So far, the best episode I have come across for this achievement is episode 5.
When you get into the building where you grab the Helicopter keys, an enemy will smash through a door. Kill or run past him, and you will find a Hunting Rifle here. Ammunition can be found throughout the episode, but as I said, it is scarce.