I don't know how hard the carbon fiber is. The arrows I'll be using have a film dip so any wear will show up on the film dip pretty easily. I'll put mine together later this morning after I dig out my arrow cut-off saw to get a precisely square cut for the butt joint. I'll use 4 glued in inserts, 2 at the joint, and one at each end for accessories including a bore diameter brass loading tip
I did mike a bunch of carbon arrows earlier this morning and most shafts run right at about .30 od. The Beman/Easton H.I.T (Hidden Insert Technology) slim-shafts mike .272 od
Modern competition rifle shooters use hardened steel rods that are precision ground to a close to bore diameter. I use them on my offhand match 22 rf single shots. Of course they clean from the breech to avoid "wearing out" the critical muzzle crown. Their theory is that softer rods that are smaller can flex, even with a bore guide, and they pick up grit that embeds in the rod. The bore guide keeps the rod centered at the muzzle and it can't hurt, especially in cleaning when we are pumping the rod in and out a lot.