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.
"Wow. Check out the comment from Shirtless Doug Fifty!" (indicating a comment left on a busty girl's Myspace page by a fat, shirtless man from Ohio that contains both numerous references to sensuality, and numerous spelling and grammatical errors.)