Hi,
It is hard to find much detailed information about English flintlock sporting rifles likely because they did not make many of them. There was not nearly the demand for them as there was for fowling guns. The earliest half stocked English rifle that I have seen was made in 1785 or later. It was by John Manton and is pictured in Neal and Back's book on the Mantons. It has a metal rib under the barrel and a scroll trigger guard a little like those found on some Hawken rifles. Any flat-faced later English flintlock would be appropriate (Durs Egg, Manton, Late Ketland). A round faced lock like the Chambers round-faced English lock would not be correct for a rifle from that period. Barrel length would be less than 34", bore >50 cal, the hardware likely would be iron and a single set trigger would be appropriate.
dave