I was thumbing through the most recent edition of Cartridges of the World, and noticed that they listed the .502 Thunder Sabre as a new entry for this edition. I had never heard of the cartridge, and after looking it up found that it is just a .50AE cartridge with the rim thinned out and cut down to accommodate functioning in a modified AR-10 rifle. These were apparently new in 2004 (and then irrelevant and more or less defunct by 2010), and so it seems to have taken a while for C.O.T.W. to register it.
The listing in the book claims that although it may seem a pointless venture as compared to other pre-existing big bore rds for AR’s, such as the .50 Beowulf, it was competitive in terms of cheaper cost for the ammo. I looked it up, and the custom modified loads, which were only ever available from R&J Firearms, cost $45 per 30rds, which is either equal to or even more than .50 Beowulf. I am guessing this is why it didn’t catch on. That, and that you basically only get 2 Hornady XTP loads, and some FMJ stuff for that caliber. I also wonder how the spent brass was for potential reloading given that the rim could be quite roughed up after firing since it was trimmed down and weaker?