We just finished prototyping our linear rail gun shooter in PIC below.
It uses two lengths of not quite as big, tubing (one on each side) to power our ram.
The hole in middle is useful for end inserts to to anchor tubing and transition to end elements. The extreme wall thickness makes it hard to prevent inserts from slipping out. We ended up having to add a nylon insert nut to our threaded H/W ends to prevent H/W from slipping out past hose clamp at extreme (3X) extension levels.
More info at this thread:
http://www.chiefdelphi.com/forums/sh...highlight=rail
-Dick Ledford