Military ammunition uses magnum primers to ensure complete and consistent ignition in cold temperatures. They also have thicker cups for pressure containment, while the entire primer is also staked into the primer pocket -- all precautions to prevent failure in automatic fire and in high-temperature climates.

Compare cup thickness between CCI 400s and 450s, Fed 200s and 205Ms, and Rem 6 1/2 and 7 1/2. The "Standard" cups are for rifles like M1 Carbine, while "Magnum" small rifle primers are for .223 and 5.56 through .22 Hornet and 22-250.