Wyatt Earp (Kevin Costner) is seen multiple times throughout the film using Single Action Army revolvers. Mid-way through the film he is seen to have selected to keep a Cavalry with an 7 1/2" barrel as his right hand gun and an Artillery with a 5 1/2" barrel as his left hand gun. It should be noted that rumors of Earp's original Buntline being used in the film seem to be erroneous, as Earp's buntline was a 12" barrel model, while the film only exhibits 7 1/2" length barreled SAAs, it could be that his other pistols were used rather than his original Buntline. Multiple other characters are also seen using SAAs throughout the film.
Colt Sheriff's Model Single Action Army (3.5" barrel)
John "Doc" Holiday (Dennis Quaid) uses a Colt Sheriff's Model SAA revolver with a 3.5" barrel and nickel finish as his weapon of choice and uses it several times in the film, including the infamous Gunfight at the O.K. Coral. Historians have proven that during the gunfight, Doc Holiday was actually using 7.5" Cavalary SAA, not a Colt Sheriff's Model. He was believed to have carried a Cavalary SAA right up until he died in the sanitarium at age 36.
Starr 1858 Army
At a young age Wyatt Earp (Kevin Costner) is seen with a Starr 1858 Army double/single action percussion revolver tucked in his belt.
Stevens 10ga Double Barreled Shotgun
The gun given to Doc Holiday by Virgil Earp is a Stevens 10 gauge shotgun right before the Gunfight at the O.K Coral and Doc uses it to shoot and kill Curly Bill Brocius. According to historians the gun was borrowed by Wyatt Earp from Fred Dodge and was infact used during the infamous gunfight.
Colt Model 1878
A Colt Model 1878 Shotgun is seen used by Wyatt Earp several times in the film including stopping Ruffians in Dodge City and killing assassins during the Canyon Ambush.
J. Stevens and Company 1878 Shotgun
One of Wyatt's deputies is seen armed with a J. Stevens and Company 1878 shotgun. Johhny Behand is also seen using one while waiting to arrest Wyatt.
Sawed-Off Shotgun
One of the assassins who shoots Virgil Earp and cripples his right arm is using a Sawed-Off Shotgun.
Remington 1875
Wyatt Earp (Kevin Costner) uses a Remington 1875 revolver early in the film and draws it to hold up a man who tries to force him to drink whiskey and goes for his gun when he denies.
Colt 1851 Navy (w/ Richards-Mason conversion)
A man named Ed Ross (Martin Kove) tried to draw his Colt 1851 Navy revolver with a Richards-Mason Cartridge Conversion on Wyatt after hearing a drunk claim Wyatt was looking for a fight. Instead of drawing his Starr revolver, Wyatt grabs a Pool ball and throws it at the man's throat. He then takes the gun for himself and practices drawing it during the Fourth of July fireworks show. Based on Wyatt's age and the time, this gun is likely an anachronism. -Close call, he was born in 1848 or so I believe so while it could go either way, it is probably an anachronism. (Wyatt Earp was born in 1848 and the patent for the first Richards-Mason conversion was in 1871 with a second improved patent in 1872. So Wyatt would have been about 24 years old when the first Richards-Mason conversion Patent was awarded.)
Colt 1860 Army (w/ Richards-Mason conversion)
Two drunkards settle an argument at the start of the film by drawing Colt 1860 Army revolvers with Richards-Mason cartridge conversions (which are anachronistic to the time). The unruly man who tries to force Wyatt to drink whiskey tries to reach for a nickel Army cartridge gun but Wyatt draws faster and makes him leave. Big Nose Kate (Isabella Rossellini) is later seen using one, which she fired some shots at Doc Holiday for slapping her, but Wyatt Earp knocks Doc out and take the gun from her.
Remington Vest Pocket
When Ed and Bat Masterson confront two men and ask them to surrender their firearms, Wyatt runs up and knocks them both out. Ed asks if it was nescessary and when Wyatt finds a derringer, likely a Remington Vest Pocket, ready to fire in one of their hands, he makes it clear that it was absolutely necessary.