As Succthis pointed out on gamespot/faqs we call this BB Farming. Usually only works when you have a team of 4 though 'cause you always get that stupid noob who picks it up. Anyways, this is how it works. Complete the objective to 99% at the very start then go on a rampage for 10 minutes. At the last couple of seconds complete the objective. Here's what you can do with a good farming session. These are some of the guys I play with on a regular basis.
There was another game where they got more kills then that, but I can't find it. Of course they were going for how many kills they could get and didn't do any of the objective, but really, what's a measly 50 xp at that point?