Adding Curve Circle

Steps:

1. Let's first rotate the barrel object so it is standing up straight. Rotate on the X-axis and align the object with the horizontal grid line so it is placed in the positive Z-axis. Make sure to apply scale and rotation Ctrl + A.
2. Add a Curve Circle Shift + A > Curve > Curve Circle.
3. Select the barrel object and add a curve modifier (Shift + A and search for curve). Hover your cursor above the Curve Object Field and tap E to sample the curve circle object.
4. As you can see the barrel is inside out, we can solve this by selecting the Curve Circle object, go into Edit Mode, press W > Switch Direction.
5. We can scale the curve S down a bit so we complete the barrel cylindrical shape. You can move the barrel object on the Y-axis to change its size.
6. Add more loops by using the Knife tool K in Edit Mode. By holding Shift to only add edge along the center of the existing geometry (center of the edges).