Set the barrel upright, or on its side, depending on which position you find easier to work with. Draw a line all the way around the top of the barrel using a black marker in the location where you want to cut it.
Turn the barrel onto its side on an unpaved area of ground. Ask another person to hold the end of it steady so that it does not roll as you cut through it. You can also place a wooden block on either side of the barrel to prevent it from rolling.
Install a wood cutting blade into a circular saw if the barrel is made of wood or plastic. If the barrel is made of metal install a carbide cutting blade, which is much stronger.
Put on safety glasses and a dust mask.
Press the trigger on the circular saw and lower the blade toward the marker line on top of the barrel. Push lightly down on the blade until it cuts through the barrel. Move the blade towards you and cut down the side of the marker line that is closest to you until you reach the ground.
Remove the saw blade from the barrel and walk around to the other uncut side. Roll the barrel until you can access the uncut area on the bottom as well as the uncut side. Repeat the process to cut the marker line with the saw until the barrel separates in two pieces and the top comes off.