While these cartridges seem to be the same, there is a difference and it matters.

by Patrick Sweeney

Gunsmith’s should have the SAAMI-published list of unsafe caliber combinations detailing cartridge and chamber combinations that can be force-fit into a rifle and the combos that ought not be there. One surprising (to some) inclusion is the .223 Remington and the 5.56×45. While they are not the same and should not be used interchangeably, .223 is generally safe in the 5.56 but the reverse is not.

How do they differ? Given dozens of chamber dimensional drawings used by reamer makers, custom gunsmiths, and arsenals across America, we can’t cover every named variant. First, the two are the same in headspace dimensions. Consulting chamber dimensions reveals some combinations might suggest that one or the other won’t work, but the overlap of bolt face to shoulder dimensions is so great that it doesn’t matter. At least, not for this discussion. The datum line and plus-or-minus of what is permitted for the shoulder location is not the problem here. They are also the same in listed chamber pressure but that takes a bit of explaining. Where they differ is in the leade, known as the throat, and how they are loaded.

The .223 Remington was unveiled in 1962 but went into production a year or two later. Originally the .222 Special as an R&D cartridge for potential military adoption, it became the .223 Remington and was used in the then-new Armalite AR-15 rifle. When Defense Secretary McNamara forced the adoption of the Armalite, requiring the dropping of the M-14, the Army resisted. Oh, they seemed on-board at first. But then they started moving the goalposts. Literally. The original “We’ll be fine with helmet perforation at 200 yards” became 300, then 400, then 500 yards.

To get that performance, the cartridge performance had to be throttled up. Armalite and Colt could not…

Read more in the November 2019 issue.

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