You could try http://www.piranha-evil.com/tm15_parts.htm
for replacement black rails. Feel free to call him, he often takes parts off and sells separately.
I did use some regular black rails on mine after I cut the shells down but had to drill a new hole and use a spacer nut in order for it to fit flush. The more you can use the existing holes though the better.
Original length TM-15 was too long to be comfortable for me, so I cut the shroud down before I had a chance to try a Carbon Fiber barrel on it. I can imagine that a thinner or CF barrel would help make the front lighter though.