I chronographed .223American eagle 55 grain fmj at 2828 fps out of a 16 inch barrel. I thought I would get 2900 fps.
The American Eagle box stated 3240 fps.
You got a valid point.
No, he does not. U.S. military 5.56mm ammunition is pressure and velocity tested in 20 inch barrels. Commercial .223 Rem ammunition is tested in 24 inch SAAMI test barrels. You are comparing apples to oranges.
In addition, the .223 Rem and 5.56mm cartridges are not identical. They have different chamber configurations, different rifling twists, different velocity specifications and different pressure specifications. The pressure is tested using entirely different equipment and procedures. The numbers are not interchangeable.