In the science of fluid flow it is well known that a constriction in the flow channel will cause an increase in velocity of the fluid and a decrease in pressure. Constant mass flow rate requires that the flow go faster in a smaller area. Velocity times pressure is a constant, so increased velocity requires reduced pressure.
A simple conical constriction is completely satisfactory for choking lead shot. Conical is an easy shape for a reamer. The barrel is bored so as to leave a little extra "meat" in the choke area. The reamer guides off the bore. There are myriad "secret/magic" reamer profiles. After reaming, the whole "she-bang" is lapped and polished.
DDA
