We have to go back to World War 1 for the answer.  The American Expeditionary Force (AEF) that went to France was involved in trench fighting.  The standard weapon of the soldier was either the M1903 or M1917 rifles, and both were very cumbersome for the point blank gunfights in the trenches.  The solution was two fold: (1) M1897 or M1912 12ga. "trench guns" and (2) pistols.  War production was still ramping up and Colt could not satisfy the demand for M1911 .45 automatics.  Both Colt and Smith & Wesson had double action revolvers that could be converted to .45 ACP ammunition.  However, there was one glitch: revolvers use rimmed ammunition for extraction from their cylinders and the .45 ACP is rimless.  The solution was the half moon clip that held three rounds.  A soldier could reload his pistol very quickly with two 3-round clips, and, as an added benefit, he could carry reloads in a small pouch without having to bother with loose rounds.  Colt cylinders were not through-bored and had a lip to stop the .45 ACP cartridge that could be fired in the pistol without the clips.  However, the fired cases had to be punched out.  S&W pistols had through-bored cylinders and could not be fired without the half moon clips.

 

Colt and S&W pistols adopted for service by U.S. forces were designated the M1917 and saw considerable service in both World Wars.  Some even turned up in Korea.  Between the World Wars, the .45 Auto Rim cartridge was developed so that these revolvers could be fired without the half moon clips.  This cartridge enjoyed modest success, is still loaded, and components are available for reloading.

 

Since that time several revolvers have been adapted for use with either the half moon or, more recently developed, the full moon clip.  They are primarily S&W brands, but there are two other U.S. producers (Ruger and Phillips & Rogers);Fabrique National (Belgium) and Korth (Germany) in the EU; and Webley & Scott in the UK.

 

1.  9mm Luger, Para, or 9x19 NATO

  S&W Model 547 (1980-1985, specifically designed to be used without half or full moon clips, six shot)

  S&W Model 940 (five shot)

  Ruger SP101 (five shot)

  Phillips & Rogers Medusa Model 47 revolver (designed to fire 25 different cartridges in the 9mm/.38/.357 class -- except .357 SIG -- and is a six shot)

  FN Barracuda revolver (Belgium -- six shot)

  Korth Combat revolver (Germany -- convertable cylinder .38 Spec/.357 Mag/9mm six shot)

2.  .38 Super

  S&W Model 627 (eight shot)

3. 10mm Auto

  S&W Model 610 (six shot)

4. .40 S&W

  S&W Model 646 (six shot)

5. .45 ACP

  Colt M1917 (six shot)

  S&W Model 1917 (six shot)

  S&W Model 25 (six shot)

  S&W Model 625 (six shot)

6. Webley & Scott .455 [converstions to .45 ACP (six shot)]