If it isn't a "plastic" gun ... use boiling water on the metal parts (remove wood, plastic etc first) a thorough rinse with several kettles will clean areas that hold grunge and grease very well...barrel included. The metal will also become hot enough that it will dry very quickly ... then use a very LIGHT oil on moving parts.. CLP (just a drop here and there) or G96. Run a very lightly oiled patch of G96 through the barrel and a dry one just before shooting.
For longer term storage ... clean properly with the boiling water and use CLP and or gun grease .. I like the Remington Gun Grease ... but haven't seen it around lately.
(BTW - nothing wrong with "plastic" guns at all - just I have never tried the boiling water procedure with one so have no idea what will happen)