In my collection are a few rounds of .38 AMU. These look like .38 Special wadcutter cartridges but are rimless or semi-rimmed. I believe they were made at Frankford Arsenal in the 1950's.
The story is that the Army Marksmanship Unit at Ft. Benning modified some Colt .38 Supers to handle this round in an attempt to duplicate the accuracy and mild recoil of the .38 Special in an auto pistol. This pre-dated the Colt Gold Cup .38 Special.
I've never fired one, but would expect it would have slightly more recoil as the .38 Super is a short recoil whereas the .38 Special Gold Cup is simple blowback.
Anyone with experience?