The Mare's Leg weapon carried by Josh Randall (Steve McQueen) is a cut down Winchester model 1892 carbine in 44-40 caliber, but the bullets in his cartridge belt are 45-70 caliber rounds used in the larger, more powerful rifles of the day. The producers wanted to use the 45-70s because they were more visually impressive than the relatively small, pistol sized rounds actually used in the 1892 carbine. The use of the 1892 carbine is itself an anarchism as the series is set in the 1870s.
After getting offered the chance to star in The Magnificent Seven (1960), McQueen found out that the only way he could do the film, which was being shot simultaneously with "Wanted: Dead or Alive", was to fake an accident or illness and get a medical leave from the series. According to his first wife, Nellie, McQueen accomplished this feat by "faking" a car crash in which he merely crashed his car into a tree, receiving minor cuts, muscle pulls, and bruises, and getting his medical leave. The series' production went on temporary hiatus while McQueen filmed "The Magnificent Seven".
Steve McQueen was hired after Jack H. Harris, who was producing The Blob (1958) gave him a glowing reference to Dick Powell (the head of Four Star Productions). Powell also asked for, and was granted, the opportunity to view a rough cut of that film.
According to the A&E Biography profile of Steve McQueen (featured in the Season ONE DVD Set), McQueen took the role of Josh Randall because his prospects for acting in movies were drying up. When the series started becoming hit, McQueen began getting more and more noticed by the film industry. So much so that director John Sturges, who'd used McQueen in the war movie Never So Few (1959) cast McQueen in the western hit The Magnificent Seven (1960), the movie that launched McQueen's rise to stardom. After making "Mag-7", McQueen began to get more and more movie offers again and decided to quit the show to pursue his film career, thus canceling the series.
Initially, the creators of the series had a hard time selling the show because bounty hunters were thought to be unsavory characters and have been portrayed as such in numerous western shows and movies. The creators overcame this obstacle by having Josh Randall give most, sometimes ALL, of his earnings to help people such as the families of people murdered by the men Randall brings in, thus making Randall a sympathetic and likable character.
The "Mare's Leg" firearm is a model 1892 Winchester (although the series is set in the 1870's) with shortened barrel and cut-off shoulder stock, a product of the era of "gimmick" guns in Westerns of the 1950's. Several firearms manufacturers produce firing replicas of this firearm and its holster. It is too short to be fired like a rifle and if fired like a handgun requires an awkward and uncomfortable angle on the wrist. Although visually striking, from a practical standpoint it is almost useless for self-defense.
There is a modern day Mare's leg. It is made by Henry Repeating Arms.The rifle holds 5 rounds and is a joy to shoot. To begin with if using this for self defense in say a home invasion it is much easier to carry around the house with you due to its shortened size. Having a green laser mounted on it, it becomes a formidable defense gun. I do not know who came up with the idea of this weapon for the series but Henry states in its catalog that it is modeled after the WANTED DEAD OR ALIVE weapon.