I realise that 'butt' has a totally different connotation (😮🙈) on your side of the pond!
So what do you guys call such a barrel for collecting rain water?
Spent some time in the States a while back; New york, Long Island, Buffalo, Boston, Philly, Chicago, Florida (just in time for Hurricane Andrew 🌪️🌩️🌧️) ...
But 'butts' (water or otherwise) weren't exactly the topic of conversation!! 🙉
Cheers, Doug 😎