On mine, the wood is tightly inletted and it can not move. If it tries to swing back, the back of the barrel will hit the wood and stop it immediately. Inletting was the hardest part of making the base. I drilled a hole to clear the trunnion, squared the sides of the hole off going up to the top with a circular saw, and then the rest was done with a chisel. My barrel weighs over 90#, so simply testing the fit every so often was a major workout.
If you are going to paint it rather than stain it, and find that you cut away a bit too much wood here and there, Bondo Fiberglass Gel will help you out. I masked the base of the barrel with celophane tape first to insure that the stuff did not stick to the barrel.