Thread: 5.56 and .223
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Old September 3, 2012, 07:58 AM   #7
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Join Date: November 2, 2007
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The pressure difference (very little) is not the main concern. The 5.56 round with a heavy (long) bullet used in a .223 chamber with a short leade can theoretically jam the bullet in to the lands and grooves causing a pressure spike.

here's article I ran across that adds info.

5.56mm cartridges are loaded to a higher pressure (usually more powder) than a 223 Remington cartridge. This typically gives you more velocity with the 5.56mm round (barrel lengths being equal) than the 223 Remington. Both fire .224 caliber projectiles.

The difference between the 223 Remington and 5.56mm NATO chambers are in the throat area (also termed the "Leade" area). This is the portion of the barrel directly in front of the chamber where the rifling has been removed to allow space for the seated bullet. The throat in a 223 Remington chamber is typically .085" while the throat in the 5.56mm chamber is typically .162". The added space in the 5.56mm chamber compensates for the added pressure inherent in the 5.56mm NATO round.

The freebore area is the space that stands between the portion of the chamber machined to the fit the cartridge neck and the tapered beginning of the rifling in the barrel. This is also the distance that the bullet ogive (the point on the bullet that first contacts the rifling lands) has to travel before engaging the bore rifling.

The Leade is the full transition area from the end of the chamber to the full rifle land diameter. The beginning of the rifling is tapered to allow a gradual entry of the bullet to full rifling diameter (in our case, .224"). This encompasses the freebore area as well as the tapered rifling area.
Since the .223 Remington chambering dimensions are tighter than 5.56 NATO chambering, this can potentially create an over-pressure situation, exceeding SAAMI specifications, if you fire a 5.56mm NATO round in a 223 Remington chambered firearm. This overpressure situation is something that will surely damage your brass and potentially damage your firearms and you! This is the reason why you should not fire a 5.56mm NATO cartridge in a rifle chambered for 223 Remington.

The 5.56mm brass cases are a bit thicker in the walls and head than the 223 Rem. This is meant to add strength to the case for the higher pressures inherent in the 5.56mm round. While the added strength better contains the increased chamber pressure, this added thickness reduces the powder capacity and can further cause overpressure problems.

Since the 5.56 chambering is slightly more spacious, it is compatible with pressure profiles from either the 5.56 and/or .223. The larger chambering does not support the .223 cartridge as well (excess Leade dimensions leading to a larger bullet jump) and will cause a reduction in round to round consistency (which inevitably reduces round-to-round accuracy).
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