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.
The
Huey door gunner
grinned and opened up on the
Viet geurillas with his fifty.