Winchester NATO-Spec Armed forces Primers forty one are the exact same primers used in the Lake Metropolis five.56mm ammunition. These primers are constructed to exacting requirements for shot to shot regularity. All primers are constantly and rigorously tested for consistency and sensitivity at temperatures and winchester 41 primers problems considerably beyond the array of usual use. Winchester assures carefully-controlled weights of primer mixtures, regularity in size and high quality, precise measurements and tolerances for anvil heights and stability in extremes of temperatures and humidity.