8X50R was the service round for the Austrians for many years, from before 1900 and WWI and after until they went to the 8X56R. The models, 05, 07, 07/12 and the aircraft variants of the Austrian Steyr Schwarzlose HMG fired the 8X50R round. It is a fairly low powered cartridge, on the order of 2000fps or thereabouts, but with a very heavy slug. I believe the Schwarzlose is the only MG ever manufactured and issued in that caliber.
The Schwarzlose is one of my favorite MGs and years ago I acquired a large supply of 8X50R from Sarco, all of which was in perfect condition even though of '35 and '36 vintage. Much of it has gone downrange by this point, but I've converted some of the Schwarzlose MGs that I have to 54R and 7.92, which helps to save the remaining supply of 8X50R.
8X50R doesn't show up much any more, so when it does the sellers ask pretty high prices.