This gun is ONLY available in Chapter 21. The gun is similar to the M79 but it is smaller and carries more ammo per clip. The reason you can only get it in chapter 21 is because it is Sully's unique gun and you can only use it during the boat scene when you take control of Sully.
To grind these kills you will want to play the chapter until you reach the first set of enemies. Kill the first guy on the right, then the two just ahead. When you turn the corner look to your right and kill the two in the distance, then turn to face the wall to your left and kill the two here. You will then reach a bit where 4 enemies will be ahead of you, kill them as quick as you can. After that you will see 3 enemies on the right hand side, kill those. Just before you reach the rope bridge kill the guy that appears to the left just before it. As soon as he dies press then 'Quit to the main menu' then 'continue'. This is also the best spot to grind for Spanish gold bounties. The trophy will unlock when you get 50 kills with it.