Pallet racks, by their very definition, are angular and made to fit square or rectangular pallets. If you import and transport several shipments of round- or cylinder-shaped objects, you may have difficulties stacking and storing them in these pallet rack bays. As such, they may be taking up floor space until it is time to ship these items back out. However, there are ways to store round items in square-ish (pallet rack) holes.
Drum bins can help you store all of those drums that come into your warehouse and place the drums within a pallet rack bay. These bins closely resemble barnyard fences in that they are comprised of a couple of boards nailed lengthwise to several vertical posts. (You might even be able to make your own if you buy a few boards and set some of your dock workers on the project.) Once you have installed a drum bin, the barrels are laid sideways and stacked in rows on top of each other. Because the bins contain the barrels all around, the barrels cannot roll to either side, cannot roll out the back of the rack nor tip forward and fall out of the front of the rack.
If you have especially long, cylindrical items, these can be placed on a log rack. The log rack is rolled into the pallet rack opening and holds all of these long, log-like items lengthwise in the space. These are especially useful if shipments of long, heavy pipe or wooden dowels come in and out of your warehouse
If you want to stack cylinders or drums in a pallet rack bay, but the cylinders or drums absolutely have to be kept vertical, then you might want to invest in containment bars. The containment bars are attached to the front and back of the pallet rack along the horizontal axis. The upright barrels or cylinders are then placed in the bay of the pallet rack. The containment bars prevent the barrels or cylinders from tipping forward or back, regardless of what shelf you put them on or what knocks into them.
Other types of containment bars are more suitable for really large cylindrical items. These bars form an "x" across the front and back of a bay opening, or you can just use one bar to contain a really large cylindrical item. If you have several shipments coming through the warehouse that are multiple sizes of cylinders, you may want to set up several bays in your pallet racks to accommodate all of these shipments.