When a MG is belt-fed, the empty shell remains in the belt link. In most German AFV's the belt and shells were led back into an empty "gurtsack".
As mentioned earlier, a big sized type of "Gurtsack" could be attached to the underside of the shute/deflector to collect the empty 20 mm, shells.
On the below picture you can see part of this device. It is the green piece of cloth below the shute,
