I was just thinking, a simple ring-shaped clip like that can probably offer very high fire rates, quick reloading of new clips, linkless feed, and little manipulation of the round between the drum and the chamber. If the propulsion method is in the gun (magnetic or otherwise), then the needles will probably be quite thin and might even fit 50 or more rounds inside that ring magazine anyway. The system will also probably be quite simple and thus robust and durable. Artistically, it looks exotic and unique from human technology.
Another idea is to just thicken the ring, increasing the number of rounds that can fit in, but leave the center open. I don't think a full drum would offer five times the ammunition capacity. By my math, a ring with thickness 20% of its radius has about 1/3 the area (35.9%) of a full disk (ie: its total diameter is 10 units, and the ring is only 1 unit thick, leaving an inner diameter of 8 units). That looks close to what he has already. Doubling the thickness from there doubles the area to about 2/3 (64%). Going on to 3x, 4x, and 5x (solid disk) gives the percentages of 84%, 96%, and 100%. (A spring-loaded drum magazine doesn't use the center core anyways:
http://www.minsky.com/portfoli/ammo2.jpg)
So the ring magazine might not be all that bad. Doubling the thickness would probably be the best balance between increasing capacity and keeping the advantages of such a system.
About the barrel: an alternative to enlarging it is to flatten or thin the top support, making it look less like a barrel. Maybe have the main ellipsoid shape end instead of extending into a tube, and have a few thin strips or rods extending out as supports instead of one thick tube.