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Case Histories: 5.56x45mm NATO

Case Histories: 5.56x45mm NATO

We’ve skated around this highly popular and influential cartridge on numerous occasions, discussing both its immediate design precursors and the NATO design that it replaced.

In our recent look at the rare .224 Winchester cartridge we examined the development process that lead to its creation. In simple terms, it was for the need to develop increased military fire-power without the burden of increased weight and to address the reduced distances of light arms engagements. Most influential had been events in the Pacific theatre in WWII and later, Korea. After a protracted series of competitions and re-evaluations of cartridges based around a .224 pill and prescribed U.S. Military chamber specifications the .223 Remington was voted the winner in early 1964. Renamed the 5.56 x 45 for NATO use it is perhaps best described as the bastard child of .222 Remington and .222 Remington Magnum. The rimless bottleneck case sharing the case head of the .222 Rem and visually close to the Magnum in appearance.


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Whilst .223 Remington and 5.56 x 45mm may look the same there are enough differences to make them potentially dangerous in the wrong rifle. Visually, apart from the headstamp, the brass is more or less identical. Unlike military variants of many cartridges, even case capacities are similar. However, the move towards heavier and steel-cored bullets in the NATO spec variants, as well as their increased chamber pressures necessitated subtle but critical changes in the chamber shape of mil-spec arms. The consequence of this morphing is that rifles chambered for 5.56 NATO will happily fire both the NATO and .223 Rem commercial ammo. However, the shorter SAAMI spec throats in commercial .223 rifles can create high pressures when used with NATO ammo, especially those cartridge designs with longer, heavier bullets. Furthermore, the higher pressures and mil spec bullets of NATO ammo conspire to considerably accelerate barrel wear. It is interesting to note that CIP proof pressure is more than10% higher than that specified by SAAMI.


The universal popularity of the cartridge means that every manual has data for it and each tool and die maker catalogues the kit. All the commercial manufacturers offer brass and a wide selection of bullet weights and designs. With regard to propellants, the list is long. As a reloading subject it is pretty vice-less, requiring only a diligent approach to powder charge weight variations, consistent neck tension and COL. Accurising the cartridge is very much rifle specific. In common with other reviewers I’ve also found that barrel length has a surprisingly high impact on velocity, as much as 40 fps per inch. My Sako gives its best with the Nosler 55-grain Ballistic Tip and Hodgdon H335. A Federal primer and 21.9-grains of H335 gets me just under 3,000 fps.


The temptation to use plentiful and cheap NATO surplus ammo has done little to enhance the reputation of the .223 Rem in terms of both accuracy and barrel life – you get what you pay for. Here it’s very much one for the Practical and Service rifle crowd; even then get a slightly over pressure batch and you will get hard extraction in a straight-pull AR15 or similar. The 223 Remington is without doubt a versatile calibre with high accuracy potential be it for target shooting or small deer and vermin control. The 5.56 NATO may not be an F Class contender but for cheap plinking and Service-style shooting you’d be hard pushed to better it and ammo is plentiful too!

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