Step 1:Go to a custom game set game mode to TDM dont change anything else.
Step 2:Important when in a public match have it hovering on tdm.
Step 3:Now go back to custom game.
Step 4:Back out of custom game and find a TDM match (This has to be fast)
Step 5:When your trying to find a match it should say trying to join Ex.13 games as soon as it says that back out into your party.
Step 6:Then it should take you into a lobby where it can fit 18 people in TDM.
(May take a couple Tries)
In the game you have your private match classes you can go back into your custom game and edit your classes with guns you haven't even unlocked and lvl them up and then do the glitch.