The 5.56×45mm cartridge, designated 5.56 NATO in alliance nomenclature, is the standard intermediate calibre for NATO rifles and light machine guns. It was standardized under STANAG 4172 in 1980 as the second NATO service rifle cartridge, after the 7.62×51mm, which holds the full-power rifle and machine gun role in the small calibre ammunition range.
The two loads in the range come from different stages of the calibre’s history. The M193 is the original US load, developed for the M16 rifle and its 1-in-12 rifling. The Belgian SS109 became the NATO standard load and needs a faster 1-in-7 twist to stabilize its heavier bullet, so the load has to match the rifling of the weapons in service.
The AR-M1F among the weapons listed for the cartridge is the folding-stock configuration of the Arsenal AR-M1, a Bulgarian Kalashnikov-pattern rifle built for both the NATO-compatible 5.56×45mm and the eastern-origin 7.62×39mm. Procurement inquiries for 5.56×45mm cartridges should name the load, M193 or SS109, and the rifles or rifling twist it is intended for.