If you go to the Light Weight Parts Thread breastroker has posted on how he makes light weight carbon fiber rails that would be a lot better than pvc. I could not link to his post but I copied the post date & time so you can find his post in the linked thread pages his post are on.
https://www.ar15.com/forums/AR-15/Lightweight-Parts-List-with-WEIGHTS-AR-15-/118-638524/?page=75
breastroker Posted: 12/27/2017 4:59:17 PM PST
For you guys who wonder how I make those incredibly lightweight AR 15 and AR308 hand guards, I start with aluminum DPMS or Clark Custom barrel nuts.
I cut the extra length off the barrel nut, you don't need a 3 inch long barrel nut. Instead of epoxy gluing the CF tube on, I drill three holes for the M4x8 titanium screws and thread the aluminum barrel nut to hold the CF on.
I buy the DPMS 7" version that no one wants and throw away the carbon fiber. It's $60 for the complete hand guard
JSE Surplus AR308 hand guard They have 14 left.
Using DPMS barrel nut
Making CF 308 hand guard
Clark Custom barrel nut started out 1.5 inches deep, I cut it to one (1.07) inches deep. Started at 1.70 ounces ended up 1.40 ounces
https://www.ar15.com/forums/AR-15/Lightweight-Parts-List-with-WEIGHTS-AR-15-/118-638524/?page=76#i7594812
breastroker Posted: 1/16/2018 4:10:13 PM PST
You can also make a very light CF hand guard using a Clark Custom aluminum barrel nut @ 1.35 ounces and add an eBay 10 to 12 inch CF tube, I throw away their heavy attachment. I then drill 3 holes and tap for M4 titanium screws about 6 mm long. Weigh is between 3.5 and 4.5 ounces depending on CF length.
breastroker Posted: 1/17/2018 12:13:47 PM PST
It actually was a buddy from the 6.8 forum who ordered over $100 worth of ultra light sanded CF that was perfect for 308 tubes. I bought 90% of his stock.
I have been able to use the tubes for AR15's by first putting one inch of the standard CF glued to the aluminum barrel nut. I then drill three holes through both the ultra thin and the std. CF into the barrel nut. Ended up much lighter than any other hand guard.
The larger ultra thin hand guard then gets a quarter inch strip of standard weight CF glues into the inner at the muzzle end. Normally you can actually grip the ultra CF tube and squeeze it one eighth inch, can't do it with the reinforcement.
But I have also used the cheap eBay tubes in many configurations. The uncut Clark BBL nut weighs 1.35 ounces, much lighter than the eBay mounting of a mil spec bbl nut and then the outer attachment metal and screws. Even with a titanium barrel nut the Clark wins out.