The medal is irrelevant.You need five kills with the weapon in one life to get credit for it. Another five kills and another credit, even if the medal is a Merciless medal.
Here's an interesting question. If you get 5 kills and the bloodthirsty and then without dying turn and net another 5 kills for a total of 10 kills no deaths does this count as 2 bloodthirsty's? I could almost swear i did that once with an AR. Not certain, and curious if anyone knows.
You are not allowed to request a sticky.