^^^^^ except I would glue veneer or canvass in there to snug it up.
I've run across three or four original percussion rifles that had cloth glued to the stock under the barrel similar to what Britsmoothy is writing about. Of course there was no way of knowing when it was done, but some of the cloth appeared quite old. BTW, the glue was applied both under the cloth/against the wood in the barrel channel and over the cloth/between it and the barrel, I think in an effort to make the cloth more water resistant and stronger.
OK, now if you want to go this route with your Charleville, then you MUST bear in mind a problem that can or WILL come up ONLY because the barrel is held to the stock by barrel bands.
Right now your barrel bands have a good snug fit, BUT if you put too much thickness of cloth under the barrel breech, at least the rear barrel band and probably the middle barrel band will not go back in place where it should, because the barrel is now higher in the rear than it should be. There is a way to ensure this WON'T happen, though.
BTW, some of what I am about to describe also pertains to either using wood veneer shims or glass bedding under the barrel breech, as well.
You are lucky you are not having to deal with the open space under the barrel breech AND barrel bands that are too loose, which is what one normally has when repairing reproduction Military Arms with barrels loose all over in the stock channel.
OK, first step is to have the barrel and bands in place and mark the tang with a magic marker on the tang, where the slot in the tang screw rests when it is tightened normally.
Then cut strips of canvas or similar cloth maybe 3 inches long. The width should be no more than about a third of width needed to go from one side of the barrel inlet in the stock, down to the bottom and back up the other side. IOW, if that measurement is 2 1/2 inches, then cut the width of the pieces of cloth at 13/16" or maybe just 3/4 inch. So let's say 3 inches long by 3/4" wide IF the measurement is about that distance. If the measurement is more, then maybe as much as 1" wide.
Now the idea is to stack those strips on top of each other pretty much centered width wise in the bottom center of the barrel channel. You keep adding more strips you notice just a bit more tension when screwing the tang screw down to the original position that you marked with a magic marker. MAKE sure the barrel bands are all the way in place each time you check. Then remove the barrel and count the number of strips in the stack and write it down.
Next you have to remove the finish in the barrel channel under the strips. Scraping may be best and is what I'd recommend, though careful sanding will also work. Roughen the wood in that area so the glue will stick.
Make sure the barrel and breech plug have a good coat of mold release.
Now here's where it gets MESSY. I would lay a coat of glue on each side of each strip and press it into the cloth with a putty knife, pallet knife or even an old butter knife. I would put that strip on a piece of waxed paper and add similar glue impregnated strips on top. I would probably stop about 3 strips LESS than the number you counted, because the added glue will take up space. Put that batch of glued strips down as close to the center of the barrel channel as you can width wise.
Put the MOLD RELEASED barrel and tang back into the stock and push the handguards into their normal rear positions. Tighten down the tang screw and screw it in right back to the spot you marked with the magic marker.
Lay the Musket on it's LEFT side so the Lock Mortice is upright and clean out ALL the glue that oozed out into the mortice using Q tips dipped in acetone. Allow the glue to cure overnight.
The next morning, remove the tang screw and barrel bands and turn the musket upside down. I sit when I do this and tap the rear of the stock against the thigh of one leg until the barrel falls free.
Then take a look at how the glued cloth shims filled up the open space. If they didn't fill up the space enough to your liking, then roughen the top glued strip a bit and add the number of glued shims you think necessary to fill it in and once again place the barrel in the stock, barrel bands to the rear and the tang screw tightened down to the original marked position. Of course clean excess glue off as mentioned earlier.
Gus