It's not surprising that people change "barrow" into "barrel" because "barrow", originally something like a stretcher on legs with shafts by which it could be lifted, is not a common word anymore. This phenomenon of exchanging an unfamiliar word to a similar sounding familiar one has been quite common over the course of the history of the language. For instance, as we saw earlier, the Old English word "goom" became "groom".
Another phenomenon favouring the understanding of "Barrow" as "barrel" is that terminal l's are often swallowed up in speech, or in some varieties of the language turned into a vowel, so some people will say "barrel" as if it were "barrew".
"Barrel" came into English from French; its ultimate origin is unknown. "Barrow", on the other hand, like most garden equipment terms, likely goes back to Anglo-Saxon, related to the word "bear" (carry).
REMINDER: March 16 "Tea and Wordlady": Bachelor for Rent: Things You Never Suspected About Canadian English. More info here: http://katherinebarber.blogspot.ca/2016/02/tea-and-wordlady-wednesday-16-march.html
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