That he used a 9mm is interesting, because Springville (formerly Hobble Creek), hosts the “Hobble Creek Wranglers” who are a Single Action Shooting Society (SASS) affiliated Cowboy Action Shooting Club.
For their competitive events, pistols must be chambered for .45 ACP, rifles must be chambered for pistol cartridges of .40 caliber or greater, and shotguns must be 12 gauge.
For their special events, the revolvers used in normal SASS events are replaced with 1911 pistols. Lever-action rifles remain in use, while only 1897-style pump action shotguns are allowed.
But, I suppose, 9mm works in a pinch.
Bullet placement, not size, is the key.