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I read the 5.56 has slightly higher pressures and that if your barrel isn't stamped 5.56 you should not use the NATO round but stick with the .223 only.

Fortunately my rifle is good for both so I can get what I want when the price is right.

Travis you may know if this is bum info or not, what do you think?

"I read the 5.56 has slightly higher pressures and that if your barrel isn't stamped 5.56 you should not use the NATO round but stick with the .223 only."

Correctamundo. But, most ARs, especially from Colt and Bushmaster, are 5.56 NATO.

"Paramilitary 5.56mm ammunition should not be fired in commercial rifles with chambers reamed to SAAMI specifications. The SAAMI chamber used in sporting firearms has a smaller diameter bullet seat, a shorter throat and less free bore than military chambers. Chamber pressures can rise dramatically when military ammunition is fired in them. SAAMI specifications for commercial .223 ammunition specify an average chamber pressure of 52,000 CUP. When military ammunition is fired in the SAAMI chamber, pressures may rise to 55,000 or 60,000 CUP! U.S. manufactured paramilitary ammunition intended for civilian consumption is purposely loaded down to SAAMI pressure levels and bullets are often seated deeper to accommodate the SAAMI chamber. Ammunition certified to meet Mil-C-9963F (the military specification for M193 Ball) is only available to U.S. government agencies and private security firms under contract to provide security for U.S. government installations. Imported ammunition of that type is generally loaded to standard military levels and should be fired only in firearms with military dimensioned chambers. To the author's knowledge, the only U.S. manufactured commercial firearms with military chambers are the Colt AR-15 series, the Ruger Mini-14 and Ruger No. 1 rifles manufactured since 1986."