Thanks, AMCIguy, excellent link but I guess my real question is why does there have to be a gap? From the document you linked:
"SCOOP DISK ASSEMBLIES.—A scoop disk
assembly is mounted on each end of the inner drum.
There are three rails on the scoop disk assemblies,
forming a continuation of the inner drum helix. Also,
two sets of gear-driven sprockets and scoop extensions
are located on the scoop disks 180 degrees apart. These
sprockets and scoop extensions transfer the rounds
from the entrance cover to the outer drum partitions and
from the outer drum partitions to the exit cover. The
entrance scoop disk has two pins that hold the scoop
extensions closed when they are activated by a cam in
the outer drum. This prevents feeding rounds into the empty space where the drum partitions cannot control the rounds."
The cam referenced is at the far end of the drum in the center of the circle and it does prevent the scoop from putting rounds/brass into that void. But, why is the void there? Why not partitions instead and use the added space for rounds?
Extracted from the above reference: Numerous other ammunition magazines that have a general DRUM-TYPE configuration have been invented in the past, and at least three types are known to be in current use. However, none are known that have the ability to independently feed two of more different types of ammunition.
One known type consists of a fixed continuous helical outer partition to guide and support the ammunition case, and a rotating "stave" inner drive means to drive the ammunition rounds, by interface with the projectile and shoulder position of the round, around the fixed helical outer portion. It is easily appreciated that such a continuous outer helical guide is both difficult and expensive to manufacture. Also, because the outer helical guide must be continuous, the inner stave guide means must stop well short of extending radially outward to the base of the round, which results in unfavorable efficiencies and subsequent higher loads and power requirements.
A second type consists of a rotating helical inner drive means (looking very much like a post-hole digging auger) which propells ammunition rounds axially, with the ammunition rounds being restrained and guided by fixed longitudinal tracks in the stationary outer drum. The apparent advantage of this type is that the multiple rows all progress very slowly toward the exit end of the drum. As the ammunition rounds move slowly, the inertia of the ammunition rounds is small which promotes the ability of the system to start very rapidly. Unfortunately, the speed and mass of the rotating center helix is quite large, which detracts from the apparent advantage obtained from low ammunition round velocity, and transmission of drive power to the ammunition round is poor due to high sliding velocities between the rotating center helix and the ammunition rounds. Further, the system is complicated because the ammunition rounds exits the drum at all radial positions, which requires an additional "Scoop Disc Assembly", or the like, to obtain a continuous single stream output at a fixed location. See U.S. Pat. No. 2,935,914, issued to B. Darsie ET AL.
A third type, which is in fact a variant of the second type, utilizes a fixed inner helix with a rotatable outer drum and longitudinal track assembly. This design eliminates the need of a Scoop Disc Assembly, but results in high round inertia.
You have to have room to allow for the exchange of ammo from the entrance/exit units to the drum scoop discs. If a partition was added, to increase the drum capacity, the hand off would not work correctly. The drum is a twin helix.