I was part of a group of guys who tested two original NRA muskets (in .30-40 cal) on the 100- and 200-yard ranges.  They were not at all competitive when shot alongside ordinary Krag long rifles.  When fitted with a sling and used in prone or sitting position, they would not hold zero.  Point of impact changed with sling tension.  That's why the old guys never bought many.  They sure look neat, though.