(CNN) -- The gun used to kill the notorious Wild West outlaw Billy the Kid 140 years ago is going to public auction for the first time -- with an estimated selling price of $2-3 million.
The Colt Single Action firearm belonged to Sheriff Pat Garrett, who shot and killed Billy the Kid at a ranch in Fort Sumner, New Mexico, on July 14, 1881.
Auction house Bonhams, which is selling the gun on August 27 in Los Angeles, described it as a "relic of one of the most important and well known stories of the Wild West."
Billy the Kid, born Henry McCarty and also known as William Bonney, was one of the most wanted criminals of American Old West -- and killed at least eight people.
Garrett had previously captured Billy the Kid, who was convicted of murder and jailed in April 1881. He was sentenced to be hanged but escaped from Lincoln County Courthouse, stealing a gun from Garrett's office and shooting two guards on the way out.
Weeks later Garrett tracked him down to the Fort Sumner ranch, where he shot him dead.
As well as Garrett's gun, Bonhams is also auctioning the gun that Billy the Kid used in his escape, among a number of other items related to the outlaw.
The Whitney Double Barrel Hammer Shotgun, which belonged to Deputy Bob Olinger, is the the only firearm known to have been used by Billy the Kid, and is expected to fetch $200,000-$300,000, Bonhams said in a press release.
A man and a legend
Also on sale is Pat Garrett's handwritten contract for publication of his book Authentic Life of Billy the Kid, which told his version of events, and is expected to reach a price of $8,000-$12,000.
The book, said Bonhams, is one of the rarest in the canon and the "foundation of the Billy the Kid legend."
Billy the Kid and Garrett shared a "mutual respect," and Kid's "hunt, capture, escape and death have become the stuff of legend," the auction house said.
The pieces are among 265 lots on auction from the private collection of Theresa Earle, who, along with her late husband Jim, is among the most highly regarded collectors of Western historical firearms, Bonhams added.
Speaking to CNN, Catherine Williamson, Bonhams director of books and manuscripts, said the collection was rare and had been gathered over decades.
A rare collection
The items had great value, she said, because of the figure of Billy the Kid, whose story has captured the attention of Americans for generations, inspiring books, movies and TV programs.
"We think that this entire collection is one of the most significant 'old West' collections ever put together," she said.
"He is a really important mythical figure in American history... there's an entire industry built up around him. Pat Garrett wrote a version of his story right after he was killed... and that becomes one of the historical bases for the Billy the Kid myth."
The rumored friendship of Billy the Kid and Garrett, who are said to have once worked together on a ranch, also adds intrigue, Williamson said.
"One turns to law and one turns to crime, and they're on these parallel paths until they intersect at the very end," she said.
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