AAROW 109 Fireworks kills

Struggling with getting my fireworks kills with the AAROW 109. I restarted a game and DID NOT pack a punch the weapon, I just equipped the fireworks mod. But I hoard up the zombies try and shoot a straggler but it seems to never work out. I have 3/75 with over a hour of trying. It either kills all remaining zombies, or doesn't activate until the last zombie which doesn't count.

Any suggestions on the easiest way to get this done?