Winchester NATO-Spec Military Primers forty one are the same primers used in the Lake Town five.56mm ammunition. These primers are constructed to exacting specifications for shot to shot consistency. All primers are frequently and rigorously tested for consistency and sensitivity at temperatures and problems considerably beyond the choice of standard use. Winchester guarantees carefully-controlled weights of primer mixtures, consistency in dimensions and top quality, exact measurements and tolerances for anvil heights and steadiness in extremes of winchester 41 primers temperatures and humidity.