Fifty caliber.. a round of ammunition the bullet of which is a half inch (12.7mm) in diameter.
A common .50 cal round is the .50
BMG (Browning Machine Gun), the round which is (unsurprisingly) designed for
the Browning .
50 caliber machine gun.
The BMG itself is the large belt-fed machine gun known as a
Ma Deuce. It was invented at the time of WWII but has remained in manufacture to this day because it is a very sound and effective design which it would be kinda hard to really improve on.
Now however there are a lot of other guns, including shoulder-fired rifles and even a single-shot pistol, firing the BMG round.
Fifty can also refer to the weapon used to fire .50 caliber ammuntion.