I stumbled across this DIY project recently for storing 55 gal. water barrels horizontally. The biggest drawback of standard upright water barrels is getting the water out when needed. This generally involves either a siphon or a hand pump. Storing the barrels horizontally and adding a spigot to one of the caps makes using the barrels much more convenient. You'll also be more likely to exchange the water every six months since it will be easier to do.