The famous gun that is used by Matthew Quigley is a replica model 1874 Sharps Buffalo Rifle, built by Shiloh Rifles, of Montana. They are chambered in .45-110, which is a .45 caliber bullet propelled by 110 grains of black powder. This round was the most accurate and powerful in the world until the advent of smokeless gunpowders.
Three of the Sharps rifles were built for the film, one to be used for shooting, one as a club in the fight scenes, and a back-up weapon in the event one was damaged. Tom Selleck requested to keep the 3 rifles built for the film. He had two of them reconditioned to remove scratches, straighten and tighten some of the moving parts. Years later, he signed and auctioned 2 of them as part of a fundraiser for the National Rifle Association, of which he is a national board member.
Quigley's uncanny ability to shoot accurately to 1200 yards and beyond with iron sights is not as far-fetched as it sounds. In the 1874 Second Battle of Adobe Wells, in northern Texas, William Dixon used a .50-110 Sharps to shoot an Indian off his horse from 1,538 yards. This is nine-tenths of a mile, and such a heavy bullet propelled by black powder would have a trajectory similar to that of an arrow fired from 100 yards, striking its target at a 30-degree down angle. Dixon claimed his shot was one of utter luck.
Sharps Rifles are now so inseparably related to this film that they are commonly nicknamed "Quigley guns." Sales for such weapons increased by over 1000% following the film's release, especially in the United States and Australia, and continue through 2013.
The film was to have been Steve McQueen's follow-up to The Hunter (1980), but he fell ill shortly after filming The Hunter (1980), so the project was scrapped. It was revived several times throughout the 1980s, with various stars in mind for the lead. Harrison Ford was offered the lead role, but turned it down because he considered it too visibly similar to Indiana Jones. The role was offered to Tom Selleck twice, first in 1985, but he had to turn it down because of his commitment to Magnum, P. I. When this series concluded in 1988, he quickly asked if the role was still available and accepted it. The producers spent another year gathering a reasonable budget.