>>64095230
It's a real cartridge, it's what OP (or the guy in the video), had his 10mm Auto revolver bored out to.

It was originally made for those short-recoil Magnum pistols by Arcadia Machine & Tool (AMT), the Automags (not to be confused for the prior Auto Mag Pistols by Auto Mag Corporation, though there were some of the same people involved). Of AMT's Automag IV series, there was .44AMP, .45WinMag, and then least common, 10mm Magnum.
For reference, the Automag II was in .22WMR, the Automag III was in 9mm WinMag and .30 Carbine, then Automag V was in .50AE, and like three or four examples of that one were made in .440 CorBon (the old forerunner to today's .429 Desert Eagle).

You could also get the LAR Grizzly in 10mm Magnum, and this would most probably be a far nicer experience than the AMTs, as their guns often have issues. LAR also made a whole bunch of different caliber conversions for their pistols, so you could get one in 10mm Magnum which then had a 10mm Auto 'trainer' kit, likewise a .45 Auto kit for the regular .45WinMag.

10mm Magnum never really caught on much at all, and that's saying a lot considering that .45WinMag and .44AMP were never big hits themselves. Of the Magnum cartridges AMT played around with, the only more obscure one would be their one .454AMT prototype, which sheared of its link on the second shot fired. That was probably because of improper heat treat, but probably for the better knowing AMT.
Anyway, 10mm Magnum is even more of a handloader's cartridge than .45WinMag and .44AMP