There are only two types of C Drums (100 round capacity.)
First, the Colt C drum: very expensive and very reliable.
Second, the WH C Drum: expensive, but not nearly as much as a Colt C drum, and very unreliable (out of the box.) These drums had many problems with the springs. Merle in Oregon can rework the drum for you and put a new spring in it. After Merle reworks them, they usually work well in semi-autos and 28's (which fire about 800 rounds per minute.) They do not seem to work well in 21s (900 RPM) even with the rework.
They are neat to look at in the gun; VERY heavy when loaded!
I would suggest buying a WW2 L drum (50 round) for the price of the C drum.
In my full auto, I am very happy to be at a 50 round limit per trigger pull. It gets expensive to shoot, and by round number 50, the target is trashed!
Just my $.02 worth.