Before you go and arbitrarily reduce charges by XX% because of something you read on the internet, you need to check a few things.
The first is the link posted by CPT. In this article you will see that in fact the 5.56 Military brass has MORE case capacity than 223 Civilian brass.
More case capacity coupled with a 10% reduction in "Start" charge is a recipe for disaster. Do NOT DO IT!
Here is a good article by Sierra on the subject.
The conventional wisdom to reduce loads with military brass is familiar to most reloaders and is generally good advice. The rationale here is that the military cases tend to be somewhat thicker and heavier than their civilian counterparts, which in turn reduces capacity and raises pressures. This additional pressure normally requires a one or two grain reduction from the loads shown in most manuals or other data developed with commercial cases. While this is most often the situation with both 308 Winchester and 30-06 cases, it is less true with the 223 brass. We have found that military cases often have significantly more capacity than several brands of commercial brass. Again, take the time to do a side-by-side comparison of the cases you are working with and adjust your load as needed. There may be no need for such a reduction with the 223. Know your components and keep them segregated accordingly.