.223 Remington was transformed into a new cartridge and designated 5.56×45mm NATO (SS109 or M855). Dimensions The external dimensional specifications of .223 Remington and 5.56×45mm NATO brass cases are nearly identical. The cases tend to have similar case capacity when measured (case capacities have been observed to vary by as much as 2.6 grains (0.17 ml)), although the shoulder profile and neck length are not the same and 5.56×45mm NATO cartridge cases tend be slightly thicker to accommodate higher chamber pressures. When hand-loaded, care is taken to look for pressure signs as 5.56×45mm NATO cases may produce higher pressures with the same type of powder and bullet as compared to .223 Remington cases. Sierra provides separate loading sections for .223 Remington and 5.56×45mm NATO and also recommends different loads for bolt-action rifles as compared to semiautomatic rifles. Rifling The Sturm, Ruger & Co. AR-556 has rifling at 1:8. Their Mini-14 rifles have rates of 1:9. Ruger's American bolt-action rifle is also in 1:8. Smith and Wesson in their M&P15 also uses 1:9. Pressures Remington submitted the specifications for the .223 Remington cartridge in 1964 to SAAMI. The original pressure for the .223 Remington was 52,000 psi with DuPont IMR Powder. The current pressure of resulted from the change from IMR to Olin Ball powder. The official name for .223 Remington in the US Army is cartridge 5.56 x 45mm ball, M193. If a 5.56×45mm NATO cartridge is loaded into a chamber intended to use .223 Remington, the bullet will be in contact with the rifling and the forcing cone is very tight. This generates a much higher pressure than .223 Remington chambers are designed for. NATO chose a 178-mm (1-in-7) rifling twist rate for the 5.56×45mm NATO chambering. The SS109/M855 5.56×45mm NATO ball cartridge requires a 228 mm (1-in-9) twist rate, while adequately stabilizing the longer NATO L110/M856 5.56×45mm NATO tracer projectile requires an even faster 178 mm (1-in-7) twist rate. Chambers The .223 Remington and 5.56×45mm NATO barrel chamberings are not the same. While the cartridges are identical other than powder load, bullet weight, and chamber pressure, a significant difference is in the barrel of the rifle to be used, not in the cartridge. The 5.56×45mm NATO chambers are dimensionally larger in certain critical areas than .223 Remington chambers. As the chambers differ accordingly the head space gauges used for the two chamberings differ. By observation, 5.56×45mm NATO ammunition