Kill Streaks

I was just wonder if any of you guys have any knowledge of the time limit between your kill streaks e.g Double to Multi. It seems as though when I get a double it feels longer than when I get 2 kills quickly. A few days ago I got an unbelievable and it was definitely longer when I got from fantastic to my unbelievable then the next kill after my unbelievable, but I didnt end up getting the +. Any ideas?

You know, you can't really time this; I mean it is like impossible without something telling you your Kill-streak just stopped.

I think they should make a Kill-streak timer that would be cool; i mean I would appreciate something like that, it would help a lot I think.

The gun you're using counts in the timer, and if you reload between kills the amount of time doesn't decrease as much as if you hadn't, I think. Snipers get the longest time between kills and shotguns seem to get the shortest time.