Dunno about the .500 S&W, but Marlin's .450 Marlin levergun is 1:20", it would make sense that the H&R is also since Marlin owns H&R and also uses Green Mountain barrels. It's also the rate of twist for the .45-70, its ballistic twin.
Maybe CW will check his .500 twist rate and report for us. How bout it CW??