Because the only remaining 8 gauge shotguns on the market were either too antique or too valuable to use, the movie's armorer, Gibbons Ltd., had to specially contract for replicas. Gunsmith Steve Karnes purchased three replica Colt 1878 shotguns and modified the barrels so that all three sets would have bore sizes equal to that of a real 8 gauge shotgun. The first two shotguns were designed to fire full-load 12 gauge blank rounds (one gun could be used while the other was cleaned or repaired from a previous film shoot.) The third shotgun was designed to chamber 8 gauge inert rounds, and was used for reloading scenes. All three were then given identical, 'aged' finishes, to make them appear old and used.
When Ed Harris reloads his Peacemaker (Colt Model 1873 Single-Action Army Revolver), he realistically uses the pistol's ejector rod to push spent cases out of the cylinder. Since few actors in Hollywood actually know how to use the weapons they fire in movies, most don't know what the ejector rod is, and are seen in various westerns reloading SAAs by opening the loading gate and shaking the spent case out.