5/16 X 18 thread for the original Enfield musket.
 
AMPCO is a durable alloy that provides more resistance to repeated hammer blows than most other metals or alloys used to make nipples. AMPCO has a greater rate of thermal dissipation to reduce erosion of the base hole to make for a longer lasting nipple. The small base hole reduces the amount of back blow or escaping gas from the burning powder to keep the pressure behind the round.