Where the barrel channel finally comes to the breech of the barrel, you must be careful to cut a gentle curve upward to the top of the stock. Where the barrel is inlet into the wood at the breech, the wood of the channel is full depth, and as you go forward, the wood along the sides of the barrel drops down to about the middle of the bore, or a little less. As you inlet the lock, this curve of wood is chiseled away to accommodate the fence behind the pan of a flintlock, or the top of the bolster of a percussion lock.
It's good you're asking questions before making shavings.