Carefull, there are rules for the importation of military devices (magazines)
The HK USC that is available for purchase here in the States do indeed only accept 10 round magazines. The magazine well is different and will not accept 30 rounders. (Actually, I though they were 25 rounders for the USC but who's counting). There are some sites out there that sell the materials to do a USC conversion (just google that) that replaces the magazine well, rear block and a few other parts to make it into an original style double stack USC. The last I'd heard, it takes about 2 grand to do the conversion, on top of the cost of the original rifle. The high cap mags are available here in the States as it is, but they will NOT fit in the rifle that is for sale in the US.