I had the same problem as you so looking for a threaded barrel for my TM HiCapa 5.1. Instead of finding what I was looking for, I found something better and less expensive than having one C&C machined.
It is called the Nineball 5.1 SAS strike kit:
http://www.airsoftextreme.com/store/...oducts_id=3067
This allowed me to add a suppressor to my Hicapa and the great thing about is that there is no weight added to your outer barrel, therefore the weight of the suppressor does not drag the outer barrel down.
This is what it looks like on one of my project guns I am currently building (picture link provided due to picture size):
http://www.airsoftbuilds.com/images/...0815042432.jpg