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.

I would be worried about the outer carbon fiber "skin" of the stock pulling away from the "grey stuff" (filler) if you start stuffing with it. They already seem a stupid design just having a shell wrapped around filler with exposed edges. Hi-Tec and Stug carbon stocks have a full carbon fiber barrel channel which would eliminate that risk

I've had the same issue Chris. I sanded the channel carefully and them sealed it with some black oil based paint. So far so good but be best to contact the maker and ask them.

Was at Southfort the other day and asked about the exact same thing. Broom handle (or similar) and sand paper was the answer. The "filler" (core or the stock) is an epoxy that bonds to the carbon shell and is non porous (so will not absorb water). They actually machine out the barrel channel after the stock has been formed.
I can't comment on other makes of stock. As stated, best to contact the manufacturer.