Winchester NATO-Spec Navy Primers forty one are the exact same primers used in the Lake Metropolis 5.56mm ammunition. These primers are built to exacting specs for shot to shot regularity. All primers are frequently and rigorously tested for winchester 41 primers consistency and sensitivity at temperatures and problems considerably beyond the array of usual use. Winchester assures carefully-controlled weights of primer mixtures, consistency in dimension and top quality, exact measurements and tolerances for anvil heights and security in extremes of temperatures and humidity.