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I searched and couldn't find this posted anywhere, so I decided to share this with you guys in case you didn't know.
To get some kills with killstreaks to add on to your current killstreak, here's what you do:
1) Get a killstreak (duh)
2) Look for enemies standing near vehicles and explosives
3) Use your killstreak to explode the car or explosive to kill the enemy

4) Voila. The game thinks you killed the enemy by shooting a car or explosive, and the kill will be added to your killstreak.
I've done this with the RC-XD by killing an enemy that was next to a car. Try it with other killstreaks like the Chopper Gunner when you only need a couple of kills for your next killstreak. This is a good trick because you are tricking the game into thinking you shot the car or explosive, instead of killing the guy with your kill streak.