The astronomical unit is a unit of length and is used in the Astronomical system of units and denoted by the symbol au.
It represents the average distance between the Earth and the Sun which is approximately 150 million kilometres (or 93 million miles).
It's used mostly to express distances within our Solar System.
The caliber is a unit of length based on the inch (imperial units) and uses the symbol cl.
It represents the internal diameter of a gun barrel and therefore more often than not, the size of the ammunition or round that gun requires to fire.
It is not always represented or converted exactly in inches so a table or "well-known" conversions are often used to depict measurements / ammo types instead.