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Col. This may help........

".223 REM / 5.56x45 NATO
.223/5.56x45 Rifle Brass



This is the various selections of all .223/5.56 Once Fired Range brass we have to offer in a variety of quantities and grades. Once Fired .223 Commercial only, 5.56 Lake City, 5.56 Mixes Head-Stamp, Nickel Plated .223/5.56 inventories updated as sorted.

New to .223/5.56 reloading? Check out these small facts that could help determine what type of brass you need:

There are a few things to know about .223 brass but all variations are desirable in their own right. NOTE: Specifications may very by manufacturer, please do your own research on your rifle prior to reloading.

The differences between .223 brass and 5.56x45 (specifically NATO mil-spec) brass is a variation in the thickness of the brass wall as well as the throat length (Leade) of the casing to compensate for higher pressure loads. The 5.56 will have a longer leade than its .223 REM comparable and this is reflected in how both 5.56 and .223 rifles are chambered. Note: 5.56 rounds can only be chambered in rifles designed to shoot 5.56x45 casings. .223 REM ammunition can also be fired through a 5.56 rifle, the small extra space created from the 5.56 Leade chamber will cause a very slight loss in pressure but is not dangerous to fire."

http://www.diamondkbrass.com/.223-5.56-Brass.html