What is the fps of an average BB gun?

200 or so

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The slowest BB gun ( NOT Airsoft or Pellet gun) that I know of is the Marksman 1010, it is rated at 200 FPS. The fastest BB gun I could find is the Crosman 2100 that is rated at 755 FPS. This gives you an average of 477.5 FPS

Pellet guns and rifles can attain much faster speeds.