Standard spec is .199". This allows using a 140 hybrid at 0.025" off the lands and just fits in (less than 0.010" clearance) in an AICS mag. If I was set on running jammed, I would have the freebore at .175-.180" as Grimstod suggested.
Edit- the above statement assumes you are mag loading. If dedicated as a single shot set-up, the standard reamer spec (.199") will still give plenty of bearing surface in the neck.[/QUOT