Interdynamics MKR
Encyclopedia
The Interdynamics MKR was an assault rifle concept of the 1980s using a 4.5x26mm Rimfire round using solid brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 bullets. Its purpose was to reach high velocity and effective ballistics with low recoil for the user to handle, even in full automatic. The 4.5x26mm MKR round was claimed to be ballistically as effective at 300 meters as the 5.56x45mm NATO
5.56x45mm NATO
5.56×45mm NATO is a rifle cartridge developed in the United States and originally chambered in the M16 rifle. Under STANAG 4172, it is a standard cartridge for NATO forces as well as many non-NATO countries. It is derived from, but not identical to, the .223 Remington cartridge...

round as well as stopping power with the use of a rimfire round supplemented the reliability of the rifle, however the MKR remained in prototype stage.
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