They just don't work well due to the design. Chinese double barrel S4
That post has pictures and gets into the technical aspects a little more.
There is more talk rehashed in this thread too: Qian Wei S-4
And here: http://www.mcarterbrown.com/forums/e...y-stinker.html
In a nutshell the design wasn't the greatest, the tolerances weren't ideal and I have never seen one work right in spite of the best efforts of many different people.