I got this on Chapter 26 you start this level with the M79 in front of you.
Just get as many kills as you can and then exit to the main menu and then continue and you will start at the start of the chapter again and rinse and repeat till you get the 50 kills.
Also at the start of this level is the Micro 9mm (just to your left when you start) so i emptied both guns and then restarted and got both pops in 10 minutes.