back to back sub contract rate of gst 12% or 18%

as per my knowledge 

section 5 of CGST Act specifically used wordings of Contact 

and as per section 2 (119) Works contract does not include sub contractor

Hence Sub contractor have to charge 18% instead of 12% (Applicable to govenment contract)

if you are meaning service tax regime ie before GST , please take a reference to Circular No 147/16/2011 -Service Tax dated 21.10.2011 where is  back to back sub contract is also considered as govt work and rates are chargeable accordingly.

But after GST subsumed all the earlier indirect laws , there is no such circular and the Govt sub contract rate has been changed from 22.8.17 to 12% only