As it happens, it's almost exactly the length of a SCUD missile.
Since Tonopah is a test & live fire range and it's reported that they have quite a museum of actual foreign equipment, this is likely a mock-up for training due to it's location in the impact area.
http://www.fas.org/nuke/guide/russia/theater/r-11.htmJust a guess, but best I can do based on information.

And don't forget, it's Nellis ranges, they don't always fly out of Tonopah to shoot stuff.
The Ranges are chock full of all sorts of targets, some fixed, some trucked in for exercises. Mobile SAM sites, working radars, troops with IR missiles, that sort of thing.
There's the 15 ft (approx) rocket that sits at the entrance to Tonopah, for comparison's sake. The link above is the overhead view..