Direct answer: Barrel tar is tar packed, shipped, or sold in barrels, especially in historical maritime and naval-stores trade. The phrase describes packaging and trade more than chemistry.
Tar was heavy, sticky, and valuable, so barrels were the obvious shipping unit for long-distance trade. Swedish, Finnish, American, and other tar producers sent barrels to ports, shipyards, merchants, and navies. Barrel markings could identify origin, grade, merchant, or inspection status.
Barrel tar appears in shipping records, old advertisements, shipyard accounts, and histories of naval stores. It helps explain how tar moved from rural kilns or pine forests into global maritime supply chains.
A barrel does not tell you the exact tar. It could contain pine tar, Swedish tar, American tar, or another product depending on the period and place. “Barrel tar” is a commercial form, not a material formula.
In short: Describes tar sold or shipped in barrels. Historically important in naval stores trade. Packaging does not prove the exact tar type. Relevant to maritime, forest, and port history.