Meer Hassan Ali on Kannoge, "midway between Cawnpore and Futtyghur":

• http://digi.ub.uni-heidelberg.de/diglit/meerhassanali1832bd2/0133

As Tim says, it's Kannauj.  

Rennell's "Bar" is probably "Barh" which is indeed about the right distance below (=downstream) from Patna.  See the map: http://tinyurl.com/o2xpr64

The archaeologists focus on Kumrahar, which is a suburb of Patna, and a long way west of Barh.  Other sites right within Patna are discussed and identified in the archaeological literature and the Gazetteers.

The Imperial Gazetteer (Oxford, Clarendon Press, 1908-1931) places Pataliputra between Bankipur and Patna, on the tongue of land between the rivers. See also here.

Dominik