The patent for the C-Mag was filed in April 1987
Here's a link to the patent document
http://www.google.com/patents?id=A6s...page&q&f=false
According to this website, the US Army tested the C-Mag recently in Afghanistan and found it had problems. Typically these were jams and failures to accept the stated 100-rds into the mag.
http://www.defendamerica.mil/article.../a072803b.html