Personally I wouldn't worry to much about the name so long as it isn't something completely ridiculous, i'd avoid naming it anything to call of duty specific personally incase you move to different games in 6 months time
Before you settle on a name, I would recommend running it through the website namechk.com. It'll see if that name has already been used on not only YouTube, but Facebook, Twitter, Twitch, and a lot of other sites. It only checks to see if the name is used in an URL so, on YouTube, for example, someone may not have the URL YouTube.com/DropShot but there may be a channel that has DropShot as its name.
I would take some inspiration from things you love within the game or with similar games. So for example, although ChargunHD is my old name, I thought of it combining two things. My name, Charlie & my favorite weapon of all-time, Shotgun.
[/SIZE] Charlie + Shotgun - Char-lie + Shot-gun - Char + gun = Chargun
Being creative is key and don't be afraid to use short names as short names are even better because they make them easier to search