If your going to go with a thinwall barrel or oil tank, you might want to line the bottom where the wood/logs are going to lay with a refractory cement. A cheap way to do it is with regular portland cement, perlite and sand. (1 part portland, 2 parts sand, and 3 parts perlite.) You can get perlite in a garden center, they use it for drainage in potted plants. It's nothing more than volcanic ash, that's why it can withstand the heat. Mix it all up so it's thick and not runny, kind of like peanutbutter, and lay it in about two inches thick. Let it dry for a few days and start a small to medium heating fire to cure the refractory. The lining will make the barrel last for a long time and not burn out in a couple of years.