the thunder 535 was designed as a prototype. it was not developed to demonstate their recoil system. which in fact, uses a nitrogen gas system. the company couldnt bring the manufacuring price of the gun down, so it never made it to production. there are about 25 of them in existence. i fiddled with one at a gun show in texas 5 years ago, a very lightweight gun... considering its almost complete titanium construction. its actualy lighter than a smith 500.
as for the comments asking why you would need a hand gun that big... have any of you ever shot a smith 500 with 440 grain rounds? its fun as all hell... its easily my favorite gun in my entire collection, even above my amt's. i can only imagine how fun a bmg pistol would be! you shoot a frozen turkey with a 320 gain hollow point smith 500 and it does serious damage... but a 50 bmg?!?!? i would pay out the ass to fire just one round!!!