I think the barrel channals in pre-carve stocks are intentionally made small, so the barrel can be "fitted" to the stock. Two barrels which are supposed to be the same size are often slightly different.
One time after getting a barrel fitted, I decided to change calibers, but found the second barrel smaller than the first, leaving a gap in some places.
Another thing, I file the two side flats, which fit into the channal, down until they are smooth, before fitting the barrel. Depending on how rough the barrel is this can narrow the barrel some. Actually, on a Colerain barrel, I once measured the narrow place before and after filing and found it 0.015" smaller than when I started.
Hope this helps,
Andy