Barrel bedding unheard of in the 18th Century, in my limited knowledge. I've seen plenty of inlets for swamped barrels done with a round plane. Bottom of the barrel channel is round.
That little sliver of wood that surrounds the barrel, AKA forestock, can barely hold its own shape without a barrel in it. In the 18th Century style of gun, the BARREL is the STRUCTURE.
Round channel for octagon barrel gun:
Breech view of same gun:
Nose section, where a fitting the octagon had taken place: