Facts: From the Ruger Website..Can I shoot 5.56 NATO ammunition in my Mini14 or Ranch Rifle? Answer: With the exception of the Mini14 Target Rifle, which accepts only .223 Rem. ammunition, .223 Rem. and 5.56 NATO can be used in all Mini14 rifles and Ranch Rifles. The chamber dimensions between the two cartridges are different. 5.56mm has a longer throat. This longer throat reduces pressures as compared to the .223 Remington. Consequently, .223 Remington ammunition is loaded to a lower pressure. The 5.56mm cartridge is loaded to higher pressures than the .223 Remington. Taking advantage of the longer throat, the 5.56mm can safely operate at a more powerful loading level.
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