The Big Circle is a caravan route in north California that loops through several settlements including Broken Hills, New Reno, Redding, the Den, Modoc and Vault City. Gecko is also included, particularly when caravans from Broken Hills (before its collapse) or other uranium producing settlements are involved.
There are many caravans on the trail, differing vastly in equipment, weapons and carried goods. By far the poorest are traders from Den, usually travelling only with two bodyguards armed with knives and submachine guns, on top of personal slaves. The Vault City, despite its wealth, typically dispatches merchant with a guard detail armed with hunting rifles, 10mm weapons and Desert Eagles, clad in metal. New Reno doesn't skimp on protection and typically outfits its caravaneers with assault rifles and automatic shotguns, although sledgehammers are also a choice. But by far the best equipped parties travel under the flag of Redding trading companies: automatic weapons, such as miniguns and assault rifles, are commonplace, as is the rare high-caliber 14mm pistol and the vintage Grease gun.
Caravans operating out of Broken Hills are well armed as well: the merchant is typically accompanied by guards carrying miniguns. By comparison, their trade partner, Gecko, sends out ghoul caravans with relatively light weapons: hunting rifles, assault rifles and shotguns.
The Big circle appears only in Fallout 2. However, while not appearing in-game, the Big Circle is mentioned by Ulysses in Lonesome Road. It is also mentioned by both Ghost and Cass in the Mojave Outpost.