Noticed that some people didn't know how to go about this so I'm posting it here. It's common to most UE3 games for those curious about it. It works on MP if you're hosting the match yourself.
- Open the file DefaultGame.ini with the note pad. You can find this file in Steam/SteamApps/common/Aliens Colonial Marines/PecanGame/Config.
- Find the string DefaultGravityZ= and change the value to a higher one. For example: -150.
- Save the file, you're done.
