Winchester NATO-Spec Armed forces Primers forty one are precisely the same primers used in the Lake Metropolis five.56mm ammunition. These primers are designed to exacting specs for shot to shot regularity. All primers are regularly and rigorously tested for consistency and sensitivity at temperatures and circumstances significantly past the range of usual usage. Winchester guarantees diligently-controlled weights of primer mixtures, consistency in measurement and excellent, specific measurements and tolerances for winchester 41 primers anvil heights and stability in extremes of temperatures and humidity.