The photo of the MG-FF above is of the fixed version. The one used in the Ju 88, in pictures I have seen, is of the flexible-mount version. This has a bulbous cover over the reciprocating yoke around the barrel, and is also fitted with pistol grips and a shoulder stock. There's a photo of it in 'Rapid Fire'.
Ammo capacity of the MG-FF was almost invariably a 60-round drum. There was a 30-round drum for flexible mountings where space was tight, and later on a 90-round drum was developed. The only belt-fed version was developed for night fighters. It used a separate electric drive for the belt, which had a capacity of up to 2,000 rounds (!!!)
Tony Williams
Author: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website:
http://www.users.globalnet.co.uk/~autogun/index.htm