Got hands like an orangutan? Big SOB's, so they are. Over 4 pounds empty with a 7.5" barrel. Performance Center thing too.
$1200 will mean used revolver. Used isn't a horrible thing, but MSRP on a Smith Model S&W500 is $1,579.00. $1,349.99 in Bass Pro. Couple on Gunbroker within your budget though. One of 'em has about 3 hours left.
MSRP on a Magnum Research BFR is $1,184.00. Also a big SOB. Almost 5 pounds empty.
"...what is a double fire..." Operator failure to follow through on the shot. Fires a second shot under recoil from releasing the trigger too soon.
As mentioned, .500 S&W is expensive stuff. Starts at $38.99 per 20 for 330 grain cast ammo. Jacketed runs $60ish per 20. Midway prices.