My rapid fire isn't very fast

Rapid fire will work as well as the specific weapon will allow it, which will vary upon games and even weapons.

example:

A single shot pistol in Call of Duty will fire much faster with rapid fire.

A single shot pistol in Halo will fire fast, but not as noticeably as the Call of Duty pistol.

Adjustable Rapid Fire will allow you to fine tune weapons firing rates, however it's effectiveness is dependent on the in game programming from the game developers.