From my new book,
Autocannon A History of Automatic Cannon and their Ammunition:
The Model 3 version of both the 20mm Type 99-1 (20 x 72RB ammo) and the Type 99-2 (20 x 101RB) could take 100-round drum magazines. The weight of full magazines was 37 kg for the Type 99-1 and 39.2 kg for the Type 99-2.
The MG C/30L was fitted experimentally to the He 112 in an engine mounting: the standard magazine for this was a 100-round drum, unusually mounted underneath the gun breech. I have no data on it.