Winchester NATO-Spec Armed forces Primers forty one are the identical primers used in the Lake Metropolis 5.56mm ammunition. These primers are designed to exacting requirements for shot to shot consistency. All primers are frequently and rigorously tested for consistency and sensitivity at temperatures and circumstances considerably over and above the variety of standard use. Winchester assures carefully-managed weights of primer mixtures, regularity in size and top quality, exact measurements and tolerances for anvil heights and winchester 41 primers balance in extremes of temperatures and humidity.