They appear to be round drums, like the Harbor Freight drums, and a number of folks use those. Drums with polygonal profiles lift the contents and drop them on each other, which is theoretically more aggressive action, so I suspect the only penalty for a smooth interior drum surface is perhaps having to run it a bit longer for the same degree of cleaning. But even that much, I don't know from experience. I've been using one of the crashing gravel sound Frankford Arsenal units for this purpose as well as a quieter older Thumbler's Tumbler, and both have polygonal drum barrels.