Like others had previously stated, it really all depends on the field, and the games objectives. I usually average any ware from 5-12 kills in a game.
HOLY CRAP dude, thats incredible 50% of the kills total, nice job. (didn't wanna make a new post)At ops where it's about 50+ I'll get around 20-30 kills a game, if I'm really on fire I'll get 30-40. Those days are rare, but majority and for safety sake I'll say I get about 25 kills a game, and I maybe get shot out once.
Now for fun little rec games, usually I kill 75% of the people on the enemy team.