It's a bulge buster, they've been around for ages. I use one for 357 SIG & 40 S&W. You can also use a .40 S&W LEE Carbide Factory Crimp Die with the guts removed. You push the cases completely through the die with a "pusher" like the ones used for push through sizing of cast bullets. BTW, the cases the guy was using in the video were playing very nicely with him. Many do not. I ended up with some 40 cases that came from a LE range, and there was a big difference between the cases that were fired in Glocks, and those that were fired in other pistols, presumably M&Ps. They're available for many calibers and do serve a very important function. Normal sizing dies cannot reach the web of the cases. https://leeprecision.com/case-condit...lge-buster-kit