For about a decade, I've been hankering for a .50 caliber revolver of some sort. I've written to both Ruger and Smith & Wesson, and requested they make a 4.2" version of:
or
or even
Ideally, one of the big name firearms companies would create a revolver with two cylinders: .50 AE for "practice", and .500 S&W for when you wanted 2000+ foot-pounds of muzzle energy.
I investigated getting this sort of revolver made by Hamilton Bowen. The bottom line is that it's possible, but the process would be extremely complicated and expensive.
I can wait.