Mk 211 Mod 0, but with a tracer component. This cartridge likely can be used in any .50 caliber weapon in the US inventory with the exception of the M85 machine gun, as with the Mk 211 Mod 0. Cartridge, caliber .50, armor piercing explosive incendiary (APEI), Mk 169 Mod 2 This cartridge is used against hardened targets such as bunkers, for suppressive fire against lightly armored vehicles, and ground and aerial threat suppression. It is generally fired either from pilot-aimed aircraft-mounted guns or anti-aircraft platforms, both produced by FN Herstal. It is identified by a gray over yellow tip. A tracer variant of it also exists. Cartridge, caliber .50, ball, Mk 323 Mod 0 Created by the Naval Surface Warfare Center Crane Division, this cartridge uses M33 ball projectiles in polymer cases instead of brass. It has a clear polymer case, with a standard brass head fused at the bottom. The Mk 323 can be fired from M2HB/M2A1 machine guns and GAU-21/A aircraft guns with the same performance. It gives a 25 percent weight savings over brass-cased ammunition and allows 40 percent more ammunition to be carried for the same weight. The Mk 323's polymer casing is applied to tracer, AP, API, and SLAP projectiles. DARPA (Defense Advanced Research Projects Agency) contracted with Teledyne Scientific Company to develop the EXACTO program, including a .50-caliber guided bullet. Videos published by DARPA show the guided bullet diverting to strike a moving target. Belt links Three distinct and non-compatible metallic links have been used for .50 BMG cartridge belts. The M2 and M9 links, "pull-out" designs, are used in the Browning M2 and M3 machine guns. The M15-series metal "push-through" links were used in the M85 machine gun. Pull-out cloth belts were also used at one time, but have been obsolete since 1945. Legal issues United Kingdom Within the United Kingdom, it is legal to own a bolt action .50 BMG rifle with a section 1 Firearms Certificate. Applications requesting firearms in this caliber are assessed by the same criteria as smaller calibers; with the applicant having to prove they have a valid reason for owning such a weapon. United States The specified maximum diameter of an unfired .50 BMG bullet is ; while this appears to be over the .50 inch (12.7 mm) maximum allowed for non-sporting Title I firearms under the U.S. National Firearms Act, the barrel of a .50