How big are the rounds that Gepard 1 uses? Well here is an image to show:
[img=http://i.imgur.com/tQt07Gf.png]
The Soviet 12.7*108mm bullet is to the right and to the left is the 12.7*99mm NATO better know as .50 BMG. The similarity of this two rounds allows the Gepard to be easily adapted to fire .50 BMG.

This is the original image from which this was cut, it shows WWII airplane automatic weapons ammunition.
[img=http://www.quarry.nildram.co.uk/WW2aircart1.jpg]

Also Gepard 3 (semi-automatic) was made to hold even larger bullets of 14.5×114 mm (.57 Cal), greatly improving it's performance, especially on ranges up to 1000m (the heavy bullet is less accurate above this distance). Truly this is one epic family of guns, appearing in several works of fiction, video games and in use by five countries.