Is this a 50AE modified to handle .510 bullets? Is it like JD's Whispers(designed to shoot heavy bullets subsonic?) Who chambers for it??

.50 AE? It's a pistol cartridge...AE stands for Action Express.

Yes, it is the 50 ae case necked up to take .510" bullets, and it is called the 50 American Eagle. I don't believe JD currently chambers for this round (BATF destructive device issues, or percieved issues), though he did make at least one barrel, as my buddy had one for his T/C encore pistol. He was shooting 50 bmg milsurp bullets subsonic, as well as cast bullets in the 600-700 gr range. The problem with the fast twist whisper bullets is they aren't friendly to cast bullets, as the fast twist tears up the bullet on engraving the lands.