You are going to laugh at this 'Bubba' solution! Needless to say, you have to have a gauge or the eye to tell when and if you got it straight. I place the barrel on the ground (not pavement!) with the peak of the bend on top and run over it with a front tire of an Aerostar Van. Usually doesn't get it all the first time, but little danger of overdoing it and finding it bent the opposite way. If it isn't yet straight, I put a plywood shim under one end or the other or both and try again. Repeat as necessary. I have been pleasantly surprised at how successfully this has worked.

OlManDow