Instructions: (Note: you MUST follow step-by-step exactly!)
1.) First off, you must not have an Xbox Live Membership to do this.
2.) Get two 48 Hour Memberships and add it to an account that previously never had gold.
3.) Redeem 1 of your 48-hour codes.
4.) Go to Private Match and load up the 50k Custom Game Mode from a file share. (I have it - send a P.M to me for my GT; don't want to give it out, Trey-arch might get me if you know what I mean)
5.) Back out of the Private Match and go to Local/System Link. (It should say you have the custom gametype/50k game loaded, if not, restart step 4)
6.) While in the System Link lobby, you must leave your Xbox on for the rest of the 48 hour membership period/until the membership turns to silver membership. (I recommend using a 48-hour late one night, then 2 days later doing this glitch.)
7.) When your membership runs out, you should still have the 50k Custom Gametype still loaded in the System Link lobby.
8.) Pick up your controller and Press (Y) and join someones session. It shouldn't let you join their session, because you still have a silver membership. A message should pop up saying "the game session is no longer available."
9.) Keeping the message on your screen, press the middle button and redeem your 2nd 48-hour or any code to your account.
10.) Now you must have a friend invite you to a Combat Training lobby, but DON'T accept the invite yet.
11.) Tell your friend to change the gamemode to the same gamemode your in, such as "Team Deathmatch" or "Free-For-All" (must be the game mode that you are in on the System Link lobby.)
12.) Now you can finally accept his invite and get in his CT lobby. When you get in his CT lobby, do NOT touch your controller, and tell him to back out and now you have host.
13.) Now you can click "exit" on the "game session is no longer available" pop up screen.
14.) Finally, change the time to unlimited, and start game and get all the kills you want. Each kill = 50,000 XP!

I realize this is hard work and may cost you to buy things, but I am in no way forcing you.
Note - This is your choice to do so if you happen to get the RRoD, then it's not my responsibility!
Have fun getting 50K per kill!