First thing to note - the Sigma 50-500's telephoto end is not really 500mm ... it's more like 465mm (a bit of creative fudging in Sigma's marketing department).
Second thing - how come nobody has recommended a Kirk King Cobra or a Wimberley Sidekick to support this lens? For a long lens like this, a gimbal-type mount is the best. If you use a ballhead, the lens is effectively balanced on top of your ball. If you are not careful, your lens might flop when you release the head and cause the whole tripod to come crashing down. If you use a gimbal-type head, your lens is suspended from the arm. It is very stable and much easier to use.