John Wayne himself said one time when asked that this was to tell when you only had so many rounds left. During that time period it was very common for someone to carry a handgun and carbine that used the same cartridge. Back then there were no carbines using the .45 Colt round...it had to do with both the rim design of the brass plus the metallurgy of the time period. Today the .45 Colt has somewhat different rim dimensions as well as better metallurgy to make carbines to use it.
The .45 Colt was the standard military cartridge (actually it was the .45 Schofield so the Army could use the same cartridge in both the Colt 1873 Single Action Army and the Smith & Wesson revolver that was also issued). The most popular round used by civilians was the .44-40 which the Colt, Winchester and Marlin carbines were all chambered for.
Due to the movie and television industry everyone thinks all Colts and Winchesters were .45 caliber but that was far from the truth. The carbines and revolvers used many other calibers...it may have been a logistical nightmare to keep well armed with all the different calibers. That's a big reason why the cap and ball percussion arms stayed popular for so long...black powder, lead bullets and caps were often much more easily sourced than brass cartridges along the frontier...plus new revolvers were expensive.
Arguably the most useful and popular firearm on the frontier was the shotgun.
