Thread: Opening out the barrel channel on a carbon stock

Hi, I've got a carbon fibre stock that I want an extra 1mm clearance round the barrel.
The barrel is fairly much a straight taper - fat but plain.
The fore end is filled with a firm grey stuff - no voids or H sections like you get in plastic stocks but I'm not sure if theres honeycomb underneath.
I want it to be fairly even inside so I can run a patch down around the barrel and clear out leaves, water and stuff.
With wooden fore ends I've hoed into it with 40 grit sandpaper wrapped round the broom handle, then finished off with finer grades just with my thumb. Is that gong to work on new fangled materials ? Any traps or tips ?
Thanks in advance.