As I know, the mini G2 is made as follow:
Cut the original G2 tube and leave a short body section as 1. in the above picture. Trim the body by lathe to make the outer diameter smaller.
Cut an AN14 Battery adapter. It's longer than needed to keep the rectangle shape intact. Using lathe again to bore out the inner diameter indicated by 2. above. (only the begining, not entire adapter)
3. Test/fit 1 and 2, then use epoxy to glue them.
4. The inner sleeve is cutted gradually. Cut a little, then try to fit. If not good, cut a little more. Do it several times to find the correct length.