Building a barrel stove with a horizontal stove pipe
At my last home one of the fireplace stove inserts had at least a dozen horizontal pipes that also had rock wool on top to baffle the heat going up, the hot air was injected low in the fire box to increase the efficiency it kind of sucked getting it started because of the baffle. Once it started the heat took all the smoke up and out. the baffle was part of what made the stove meet Oregon’s efficiency standards so it can work and although tough to start it may work better for your plan on heating rocks stacked around the back of the drum.