5.56x45 is the Millitary equivilent of .223 remmington.
Just like 7.62x51 is the millitary equivilent of 308 winchester.
It has almost identical external dimensions but differs in the chamber shape and the primer will be crimped into place. Hence the term "Millitary Brass".
Yes it is safe to shoot in your .223.
Check out Wikipedia's explanation.
.223 Remington - Wikipedia, the free encyclopedia
Cheers
Pete
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