Winchester NATO-Spec Armed forces Primers forty one are the same primers Utilized in the Lake Metropolis 5.56mm ammunition. These primers are constructed to exacting requirements for shot to shot consistency. All primers are continually and rigorously tested for consistency and sensitivity at temperatures and situations winchester 41 primers considerably past the choice of standard use. Winchester assures cautiously-controlled weights of primer mixtures, consistency in measurement and excellent, exact measurements and tolerances for anvil heights and stability in extremes of temperatures and humidity.