Most here are correct. It is OK to shoot the 223 in the 5.56 Chamber but the 5.56 NATO Ammunition should not be fired in the 223 Remington Chamber. **You are correct "Due to pressure reasons"!** The 5.56 Chamber has a longer lead not to mention the rule is that Military Ammunition normally has higher pressure than the normal commercial ammunition. So since the 223 chamber has a shorter lead than the 5.56 NATO which some also call Free Bore (Lead!) This is the distance between the bullet bullet in the chamber and when the riflings start in the bore. With loading the 5.56 round into the 223 Remington Chamber there is much less lead which causes a spike and significant increase in pressure. *Unsafe! So simply reminding once again, that the 5.56 round has higher pressure to start with so the 223 Remington Cal. ammunition can be fired safely and with good results in the 5.56 NATO Chambered Rifle. But the 5.56 NATO ammunition should never be fired in the Remington 223 chambered rifle! Most all new AR Rifles today like Bushmaster, Stag, Rock River, DPMS, Spikes, LMT, Daniel Defense and other good ARs even though they may make reference to the 223 cal. The barrels are in fact 5.56 NATO or Wylde Chambers. The Wylde Chamber is simple a 5.56 NATO Match Chamber developed by Bill Wylde and will shoot both 223 and 5.56 with no problems!
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