They are not made by the manufacturer mine is from any longer.
This was a bolt and extension made for but sold by:
http://www.ar15performance.com/
It was named 'The Super Bolt' and had a .473 bolt face for short .308 case cartridges such as the 6mm BR. It is no longer offered and hasn't been for a few years. I'm sure that someone probably hotrodded this setup and ruptured the case and bolt, effectively putting an end to a somehwat fun project. I used the 6.8 SPC magazines with some slight modifications.
Lacking any of the above, I certainly would try the .30AR and be happy. You might have to have it made for you but that's certainly doable these days.
Best.