Anyone on the 'fuge here?
That is indeed the million dollar question, just about everything I have read says .721. I had mine miced today at Briley, .721. They are building me a X2 LM with the same THREADS as the X-gun, but for a smaller bore.
I think the chokes for the Sport II will fit the SBE2 and have the same bore but I haven't had a chance to test them yet.