Is this an external cannon on an F-16?
What you seem to have found is the Litening Targeting Pod. 


  Northrop Grumman's widely fielded LITENING system is a combat proven,
  self-contained, multi-sensor targeting and surveillance system.
  LITENING enables aircrews to detect, acquire, auto-track and identify
  targets at extremely long ranges for weapon delivery or nontraditional
  intelligence, surveillance and reconnaissance missions. Source


pod http://media.defenceindustrydaily.com/images/ELEC_LITENING-AT_on_F-16C_lg.jpg

The F16 does have a cannon as well, but it's located on the port side under and aft of the canopy: