Not only are these two rounds often interchangeable and able to be shot from the same gun, their history is entwined with each other. In fact, the 5.56 NATO round didn’t come about until have the .223 was produced. In 1957, Fairchild Industries set about to create and new lightweight combat rifle. They were commissioned, along with several other groups of engineers, by the US Continental Army Command (CONARC) with a set of specific criteria that the gun and round had to meet.
These parameters included:
• .22 Caliber
• Able to exceed supersonic speeds at 500 yards
• Rifle itself could only weigh 6 lbs.
• Able to penetrate both a steel helmet and .135” steel plate at 500 yards.
• Magazine capacity of at least 20 rounds.
These specifications came as a result of the 7.62 NATO round being deemed has having too much power to be carried by infantry. In May of 1957, Eugene Stoner shot the first AR 15 design to CONARC. Through further developments, the AR-15 was transformed into the M16 rifle that is still the standard rifle for most of the American Armed forces.
Each box contains 250+ once fired brass shell casings. May contain some 5.56 mixed with 223.
This is unprocessed brass sourced from commercial shooting ranges. It has not been cleaned or resized and still contains the spent primer.
This brass contains different manufacturer's headstamps. It is recommended that all brass be inspected prior to being reloaded and fired.
WARNING
– This product can expose you to chemicals including lead and lead compounds,
which are known to the State of California to cause cancer and birth defects or
other reproductive harm.
For
more information go to
www.
P65Warnings.ca.gov
and see: Warning
Scam!!!! 
Good brass. 50% 5.56 
Very nice