The original lock geometry of the 1803 Harper's Ferry rifles is hard on modern English flints, at least...don't know what the results were with the French amber flints originally issued with the rifles. Many years ago, "Doc" Carlson at Crofton, Nebraska, (Upper Missouri Trading) identified the problematic angles and modified a number of cocks for the then-imported Navy Arms replicas. It cured the problem; the modified hammer was not noticeably different than the original HF cocks from which the Navy import was copied. (There may have been a short article in Muzzleloader or Muzzle Blasts at the time on this modification; I don't recall.) A number of the members of our Lewis and Clark group had these Navy imports; the modified ones worked beautifully while the as-imported ones continued to turn English flints to gravel rather rapidly. We had no source for French flints other than archaeological finds at the time, sadly.
At any rate, the popular gossip at the time was that the Navy imports were copied very faithfully from an original Harper's Ferry rifle lock, and theoretically therefore replicated not only the lock but its inherent problems when using English flints. There is a difference not only in the material but in the knapping technique between English and French flints; the original HF lock was designed to use French flints.
