Even though they appear to be virtually identical, the commercial .223 Remington and its military counterpart the 5.56mm NATO cartridge are as different from one another as apples are from oranges. The essential differences, however, are not based on the cartridges’ case dimensions — for they are exactly the same — but rather on the ballistic parameters that the metallic look-alikes were designed around and the barrel chambers for which the rounds are reamed.
Chambers for these prolific rounds are offered in three guises: the Mil-Spec 5.56 in military and commercial M-16 rifles and M-4 carbines; the conventional .223 Remington for commercial AR platforms, long guns and handguns; and the .223 Match chamber used by serious competitive AR shooters.
Let’s start with the ubiquitous .223 Remington first. These precise chambers are cut to SAAMI (Sporting Arms and Ammunition Manufacturers Institute) specifications, not Mil-Spec (military) dimensions. As such they are slightly smaller and tighter than Mil-Spec chambers. While many perpetuate the myth that it is perfectly safe to fire the visibly similar 5.56mm rounds in .223 Remington chambered weapons, this is definitely not the case.
Because it is designed to function in a wide range of selective-fire and fully automatic battlefield weapons, Mil-Spec 5.56 ammo is loaded to higher pressures than commercial .223 loadings. Couple this with the fact that the substantially higher pressured 5.56 is corralled into the smaller, tighter .223 chamber and we set the stage for some serious pressure escalations and potential safety hazards. Additionally, 5.56 rounds are often loaded with heavier weight bullets (62-77+ gr.) than their commercial .223 counterparts. The longer throated 5.56 Mil-Spec chambers easily accommodate these lengthier slugs, but loading these heavyweights into a shorter throated .223 can cause the bullets to be tightly jammed into the rifling. Upon firing, dangerously high pressures can be generated as the propellant gases attempt to drive the jammed bullet down the rifled tube … again, not a good scenario.
Blown primers, failures to extract and, in extreme cases, cracked uppers have convinced many informed .223 AR shooters to steer clear of the readily available and often attractively priced surplus and imported 5.56 ammo. Again, you can fire 5.56mm ammunition in your .223 Remington AR or long gun, but it is certainly not advised.
Unfortunately the situation can be further clouded by the fact that some ammunition and AR manufactures print both caliber designations (.223 Remington/ 5.56x45mm) on their ammo boxes and AR receivers. The only way to know for sure how your AR is machined is to check the inscription on the barrel. It will be clearly denoted; then you can make the proper ammunition selections.
If you’re currently in the market for a .223 caliber AR or long gun and are not interested in a purpose-built target/competitive weapon, then it makes perfect sense to select a 5.56 chambered model. You can utilize the full range of commercial .223 Remington ammo, have access to truckloads of surplus and commercial 5.56 ammunition, and experiment with those high ballistic coefficient heavyweight projectiles on long-range targets and game; you can reap the benefits of both worlds.
45mm chambered fire-arms are the most popular centerfire weapons in America today, particularly in their AR guise. Select the model and chambering that best fits your individual needs, then head out to the target range, varmint fields or gravel pits to exercise and enjoy your precious 2nd Amendment freedoms.
Rich Simpson may be reached at firstname.lastname@example.org.
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