Wasn't gonna mention it since it isn't really what Yuxi was asking for, but since everyone is going off on tangents...
Eagle does make a shoulder-slung low-profile MP5 mag carrier
http://www.eagleindustries.com/ProdD...p?PartNoID=248
Otherwise I'd be looking at a very compact MOLLE chest rig, or something like the BW Zip Rack... pop on two 3xMP5 shingles and whatever else.