SR300 is supposed to be capable of 60 fps at varying resolutions up to 720p.
However, the camera exposure is handled automatically by the drivers so whenever the device is used in a medium to low light environment, the FPS drops dramatically no matter what it is set at. However, if I point the camera at a light source, for example my laptop screen or a lamp, the FPS rises back up to 60 regardless since the exposure setting will not be as high.
How can I force 60 fps regardless of setting/situation?
You can't. That's how physics work.
If you need to expose for a second for example, you will get at most 1 frame per second.
If you mean disabling auto exposure, sure, you can do that, and set your own exposure time.