Assault Class Challenge

"Get kills while using the adrenaline injector" is bugged, at least on xbox. I've been stuck on 29/30. Keep getting kills after using the injector but never get credit for it. I've recorded clips getting kills after using it. I know it only lasts 8 seconds and has a 2 second activation time. I've watched the clips back and started a stop watch until I get the kill. Got a double kill at 4 seconds after pressing to activate (didn't wait to start time after the 2 second activation). Got no credit.