I heard some guy talking about having some chinese made 120 round drum, presumably for an AR-15.  Has anyone ever heard of these?  

Yes...I believe Keng's Firearms imported these years ago. They are steel drums with a deep blued finish. Looks like a 100-rd AK drum with a AR15 mag feed tower afixed on top. Though cheaper than a preban Beta C-Mag, it is heavier, especially when loaded with 120 rounds. (Remember that in an AR, the mag catch is the only thing supporting the weight of the loaded magazine) Due to its design, it's also not practical for any type of bench shooting or prone shooting. The C-Mag has a much more shorter profile, which is conducive to bench and/or prone shooting.