Rofl I'd just think it'd be hilarious to have, UNC has a drum mag for an MP5 ... what if you put that on an MP5K? rofl

Warmongers back in the day...
Apoc would carry 2-3 Cmags on him, I had one, shagg had one, Quantis tore on apart to make a G36 box mag...more I think.

Do the BBs shake inside a drum mag like they do in a hi-cap?

yes, however, you can slightly insulate it so you only hear the BB on BB contact instead of the BBs against the side of the wall.
when it winds, it is loud...like an automatic BB loader

Not always. My MAG box for the M249 is very silent when winding.

I thought C-mags were originally designed for M-16 variants to help cut down on mag change time and to reduce reload time with loose ammo? So what's unrealistic about using them (besides their capacity?) Unless you're playing with realcap limits then what's the difference between running with a couple hicaps vs. a C-mag? I can usually play several skirmishes on a 450 hicap for my G36 or my M4 so I'm rarely changing mags in a game anyway. If you want realism you could throw 100bb's in your C-mag then it would be realistic. Although, to be 100% realistic you'd have to try to recreate the jamming problem that they've been reporting from what limited field use they've been getting. :)

Actually that last part wouldn't be a problem as my C-15 has been jamming regularly. :(

yah, well, lets face it, some people take a shit if you load 33 rounds into a star mag instead of 30.

It all depends on the field limitations.

We had a guy here who showed up with an AK drum mag. After about 20 min of shooting he managed to fry his gearbox and ban himself from future ops.