it's a plastic woven braid you can buy and often find in clear water lines for kitchen sink sprayers and hookup lines.
It works well over bolt action and other 3/8" tubes as well. No casting required. Just paint your tubes, cover with epoxy, insert and pull the braid tight. Wipe off the excess epoxy and let dry before trimming. I think the texture feels great to write with also.
Looks good, and great match with the component plating!