Winchester NATO-Spec Navy Primers 41 are the same primers used in the Lake Metropolis 5.56mm ammunition. These primers are built to exacting specs for shot to shot consistency. All primers are frequently and rigorously tested for consistency and sensitivity at temperatures and situations significantly further than the array of winchester 41 primers standard use. Winchester assures carefully-controlled weights of primer mixtures, regularity in size and top quality, exact measurements and tolerances for anvil heights and balance in extremes of temperatures and humidity.