.223 Winchester Super Short Magnum
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The .223 WSSM is a .224 caliber rifle cartridge created by Winchester and Browning based on a shortened version of the Winchester Short Magnum case. Wikipedia
Base diameter: .555 in (14.1 mm)
Parent case: .300 WSM
Produced: 2003–present
Rifling twist: 1-7 in (180 mm) to 1-9 in (230 mm)