According to
oilhistory.com:
"Before becoming a container for petroleum, the 42 gallon wooden barrel was in use for wine, beer, whiskey and fish."
So, just blame 75% of the confusion on alcohol. ;-)
It is my understanding that in the late 18th century barrels were originally used to ship fish to the "interior", e.g. western Pennsylvania. Farmers there had an inadequate local market for their produce and wanted to sell grain to the coastal cities. The most compact form of their product was whiskey, which they distilled and then shipped. They used the barrels that fish had been shipped in, and to eliminate the fish taste from the barrel, they charred the inside. This is supposedly the origin of that practice. Being so dependent on the sale of whiskey for their livelihood, they were of course justifiably incensed when congress imposed a tax on whiskey. Thus the whiskey rebellion.
The part about the double use of the barrels, and the origin of the practice of charring the inside, comes, if I remember right, from the standard American history text of Morrison and Commager.