You will need to load the shooterenging.ini file which is located here:
C:\Users\ your user name here\Documents\My Games\UnrealEngine3\ShooterGame\Config\ShooterEngine.ini
Now find the following and edit it to your desired settings;
[Engine.Engine] bSmoothFrameRate=TRUE //unlock MinSmoothedFrameRate=120 //or your min MaxSmoothedFrameRate=300 //or whatever you want
Save the file. Then right click on the file and make it read only. This means Dirty Bomb will not override it when it loads the game.