Something like this actually exists, it's made by KSC for their MK23 series GBB
http://www.uncompany.com/pageproduct...sp?prodid=1407
Inside is an inner barrel retained by a spring mechanism.. the spring keeps the inner barrel inside the silencer aligned with the inner barrel on the gun once installed.
I dont' have a picture of the inside but have sold a few of these with very positive results reported from the buyers. Unfortunately it only comes in 16mm+ thread for the MK23 series.