I use a router, a Workmate work bench, a couple of pieces of angle iron that will clear the handles on your router, and some “C” clamps, and a 3/4” carbide round bottomed router bit. With the area to receive the barrel channel clamped in the Workmate, and a router bit in your router that has a point on it, adjust the angle irons so you have a track for your router, and clamp them down with your “C” clamps, make sure the point of the bit is on the centerline for your inlet. Remove the pointed bit, and replace it with the carbide 3/4” round bottom bit. Don’t try to cut the channel in a single pass. Depending on the type of wood it will take three or four passes usually. Long barrels usually require some creative moving of the stock and reclamping, but it works well. I have done several using this method, and haven’t ruined a stock yet. I wish I could say the same for the long bit I use to drill the ramrod hole. Good luck.
Hungry Horse