ford8nr wrote:
It's my understanding the 5.56 chamber has a longer leade which actually reduces chamber pressure.
This is correct, the leade in a 5.56x45 chamber is different from a SAAMI .223 Remington chamber. The difference is tiny, but allowing the bullet to travel a short distance before the rifling has to start machining grooves into the bullet does moderate the pressure spike.
When people started wringing accuracy out of the 5.56 by cutting tight short leade chambers, manufactures changed the cartridge to the .223
The .223 Remington predates the 5.56x45 NATO cartridge, not vice-versa.
See
https://en.wikipedia.org/wiki/5.56×45mm_NATO#History
1957: The development of the cartridge which eventually became the .223 Remington (
from which 5.56mm NATO would eventually be developed) was intrinsically linked to the development of a new lightweight combat rifle.
...
1962: In the spring of 1962
Remington submits the specifications of the .223 Remington to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI). In July operational testing ends with a recommendation for adoption of the AR-15 rifle chambered in .223 Remington.
[8]
1963: September
the .223 Remington cartridge is officially accepted and named "Cartridge, 5.56mm Ball, M193." The specification includes a Remington designed bullet and the use of IMR4475 Powder which resulted in a muzzle velocity of 3250 feet per second and a chamber pressure of 52,000 psi.
[8]
In 1977, NATO members signed an agreement to select a second, smaller caliber cartridge to replace the 7.62×51mm NATO cartridge.
[10] Of the cartridges tendered,
the .223 Remington (M193) was the basis for a new design created by FN Herstal. The FN created cartridge is named 5.56×45mm NATO with a military designation of SS109 in NATO and M855 in the USA.
[11] [Emphasis added]