Place the barrel on its end and measure its height. Mark the center point at 10 or 12 locations around the barrel with a pencil.
Connect the marks to form a continuous line around the barrel. Use a flexible ruler or straight strip of metal. You can use any item that will bend enough to accommodate the curve of the barrel as a guide when making the straight line around the barrel.
Turn the barrel on its side. Cut though the barrel at the line with a circular saw. Move the saw forward along the line while rolling the barrel to keep the saw position at the top of the barrel. Continue cutting all the way around the barrel.