This was a long range target cartridge (AMU = Army Marksmanship Unit). According to available information, the staff and facilities at AMU were utilised to help develop and improve sniper weapons, systems and tactics, but as far as could be ascertained no AMU cartridges were used for anything but competitions. This cartridge is for all intents and purposes the same as the 300 ACKLEY MAGNUM IMPROVED 40°. The 300 Ackley Magnum started life as a wildcat and there are many versions and variations based on a variety of cartridges (300 Win Mag, 300 Weatherby, 300 H&H etc. as well as different lengths). What makes it more confusing is that different wildcatters did virtually the same thing and called it different names.
The first two specimens are the REM-UMC 300 variations. The third cartridge is the R-P 300 and the last one is the WCC60 cartridge. As can be seen, the last two has a case length about 1mm longer than the REM-UMC version
Box with WCC 60 headstamp that was featured in the December 2015 Wards Auction
Box with REM-UMC headstamps. (IAA Forum)