Head into custom games and select the gametype you would like to use. Then go into game options and edit the game the way you would like it. Once you are finished, press X to save and name your gamemode.
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The Custom Games menu is selectable from the main menu screen. "Game Options" are available for every gametype, and variants of a gametype may be saved on your hard drive for later use.
Find a rocket gametype in someone's fileshare or play a game of rocket race online. Then save it, and edit the settings to your preferences. Voila.
You could take the shields off and put the damage multiplier to 300% and melee up to 300% percent damage and bottomless clip.