Definition of "magnum"

magnum

/ˈmæɡnəm/

noun
plural magnums or magna
1. A bottle containing 1.5 liters of fluid, double the volume of a standard wine bottle.
  Quotations
  So please don’t ask me when / There may be wine and roses / And magnums of champagne
  2014, Leonard Cohen (lyrics and music), “A Street”, in Popular Problems
2. (firearms) A powerful firearm cartridge, often derived from a shorter, less powerful cartridge calibre that uses the same bullet.
3. (by extension) A handgun that fires a cartridge of this calibre; chiefly a revolver, but rarely an autoloader firing an unusually powerful calibre.
  Quotations
  I know what you're thinking. "Did he fire six shots or only five?" Well, to tell you the truth, in all this excitement, I've kinda lost track myself. But, being as this is a .44 magnum, the most powerful handgun in the world, and would blow your head clean off, you've got to ask yourself one question: "Do I feel lucky?" Well, do ya, punk?
  1971, Harry Julian Fink et al., Dirty Harry, spoken by Harry Callahan (Clint Eastwood)