Back in the late 50's and early 60's the Army Advanced Marksmanship Unit ( now known as the Army Marksmanship or AMU) was concerned with the major problem of proper functioning of the rimmed .38 Special cartridge through converted 1911 pistols. The major problem was feeding-there was a noticeable difference in the position of rounds in the magazine between the first and last rounds. To overcome this problem magazines were modified, feed ramps were polished, and the chambers enlarged to allow the cartridge to ride up the ramp and into the oversized chamber. This oversized chamber caused problems with accuracy.
To alleviate the feed problems Herman Gano, an AMU gunsmith came up with the ideal of the .38 AMU cartridge. The .38 AMU is basically a .38 Special with a semi-rimmed head, like the .38 Super. When used in appropriate guns, nearly all the feeding problems were eliminated. The first cases were hand made by turning down a .38 Special case in a lathe to remove the rim and form an extraction groove. Later the Army contracted Federal, Winchester-Western and Remington to produce the .38 AMU cartridge.
Colt Industries followed through by producing a conversion kit to be fitted on both the 1911 and Colt .38 Super pistols. The conversion kit consisted of a .38 AMU slide with all the innards, recoil spring, magazine, barrel bushing and a .38 AMU barrel. One interesting note, other than ID markings, there's no difference between the .38 AMU and .38 Super slide. The barrel is unique in that it had only one forward locking lug so the gun wouldn't lock up too tightly and hinder proper cycling with midrange ammo. The work required to "convert" a pistol to .38 AMU is approximately the same as a full accuracy job. The slide must be fitted to the frame, the barrel must be fitted to the slide, and the barrel lug fitted so it cams with the slide stop pin.
.38 AMU never really took to the shooting crowd for several reasons. There were no empties available for reloading, (most shooters were geared for .38 Special), the conversion kits were somewhat difficult to install by the shooter, the advent of the .38 Special Gold Cup magazine which solved the feeding problem with regular .38 Special wadcutters, and the introduction of the Smith and Wesson 52, an out of the box match gun.
