25.8 TAC and the 60 NBT got me 3110pfs ina 17" Kreiger tube.
25gr Benchmark and the 60gr V-max got me 2963fps in a 16" Colt tube.

As always these loads should be worked up, yada yada.

about halfway down; http://www.thegunzone.com/556v223.html

here's another source; http://www.6mmbr.com/223rem.html

".223 Remington vs. 5.56x45--Chambering and Throat Considerations
Is the .223 Remington the same as the 5.56x45? The answer is yes and no. There ARE differences between the .223 Remington as shot in civilian rifles and the 5.56x45 in military use. While the external cartridge dimensions are essentially the same, the .223 Remington is built to SAAMI specs, rated to 50,000 CUP max pressure, and normally has a shorter throat. The 5.56x45 is built to NATO specs, rated to 60,000 CUP max pressure, and has a longer throat, optimized to shoot long bullets. That said, there are various .223 Remington match chambers, including the Wylde chamber, that feature longer throats. Military 5.56x45 brass often, but not always, has thicker internal construction, and slightly less capacity than commercial .223 Rem brass."

I don't have a strain gauge and can only go by what I've read from several sources, if you can prove otherwise I'd be interested. And yes there is some reports of different methods of testing but if you ever chronograph real NATO spec ammo (not XM) vs commercial it does run higher velocity or has in what I've seen.