Ive been doing a little testing with the cast Performance 440gr. bullet in the 500S&W. Every one making a load with this bullet is advertising a velocity of 1625FPS out of the S&W revolver. However, OUr test gun rotates the cylinder backwards when firing this load. Thats right, the gun goes bang and the cylinder rotates back wards one chamber, so when you cock the gun the next time, the hammer falls on an already fired chamber--weird. The recoil causes this. WE had to back the velocity down to 1550FPS to get the problem to go away.
I let everyone that wanted to shoot the .500 in Raton. Depending upon how the gun was held, it would rotate the cylinder. When it did rotate, the cylinder went backwards for righthanders, and advanced for lefthanders like me. It could, for a lefthander, possibly fire the next cartridge. It never happenned, but a full-auto .500 is spooky! For some shooters, the cylinder would not rotate at all. Shooters who let the gun recoil straight up had no problem, but the tighter the gun was held, the more severe the problem. S&W is aware of the problem, and hopefully have a fix for it. It might be as simple as a stronger bolt spring. As you stated, the problem only occurs with the 440 grain load, which we chronographed at 1662 fps.
I've got several hundred rounds of 625 grain loads at 1200 FPS through mine with no ill effects, but this load is about 40,000 CUP, same as a full power .44 mag. If it turns out the 500 will tolerate extended shooting with loads that are only TWICE AS POWERFUL as a full .44, I can live with that.
S&W is aware of the problem, and hopefully have a fix for it.