Ammunition feeding for machine guns is a fascinating study in itself. Besides the various box and drum systems, may things have been tried, with varying degrees of success.
The Lewis gun, and the Soviet Degtaryev (sp?) use a flat "pan" that looks like a drum, but isn't, even though its often called such.
Some Hotchkiss designs (and their Japanese clones) used a feed strip, a long (usually) brass "frame" holding the rounds. Fed in one side of the gun, and exiting out the other when empty. Some guns even put the empties back in the feed strip!
One Japanese design used a hopper, with the standard rifle 5rnd stripper clips stacked in it. (for a variety of reasons, not very successful).
Early designs from Maxim and Browning used cloth belts, and the Browning designs successfully adapted to metal disintegrating link belts. In these guns, rounds are pulled rearwards from the belt, then fed forward into the chamber.
The German MG34 &MG 42 use non-disintegrating metal links belts. Rounds are pushed forward through the belt link into the chamber. The links stay fastened together.
More modern designs generally use metal disintegrating link belts, with rounds being fed forward through the links into the chamber.
As long as not damaged, both the cloth belts and metal links are reusable for a while. Belts can be made up by hand, but using a linking machine is much better (and easier).
Don't know the specifics on the HK belt system, sorry.
The US legal definition of anything that fires more than one shot with a single trigger pull is "machine gun". No matter what else it does, or doesn't do.
The commonly used definitions in the firearms community are:
Submachine gun: full auto firing handgun ammo (can be handheld or shoulder fired, or even mounted-which is extremely rare)
Automatic Rifle / light machinegun: Shoulder or bipod fired, uses full size rifle ammo, usually magazine fed. These designs place greater emphasis on portability than on sustained fire.
Full size rifle round, belt fed, maybe fired off bipod or tripod.
Medium or General Purpose machinegun:
may be the same gun as the light machine gun, but usually tripod mounted and better adapted to sustained fire.
Heavy Machine gun:
either intended for sustained fire (such as water cooled) or classed as heavy due to caliber size (.50 BMG, etc.)
These are rough guidelines, and many guns have considerable overlap in use and features.
All else being equal (and it almost never is) bigger bullets tend to work better.