I wanted to add this comment, there is a world of difference between a factory chamber and a custom Minimum SAAMI Match chamber with minimum throat and freebore. Even 40x Chambers are huge as a general rule of thumb.
We have seen super velocities and accuracies cut from Minimum spec chambers in Varmint and big game caliber, even when Stainless factory barrels were re-chambered when new.
Shooting the 40's at 4000 is no big deal, but it does take specific things to accomplish it, minimum chamber, little longer barrel length, tough brass such as IMI or Lapua, N133, and tough cup primer, 1-14 twist barrel.
4200 out of the 223 AI with the same data above again is no big deal.
N133 is magic in speed in conjunction with extreme accuracy, usually 150 fps faster than benchmark.
These stats are usually contemplated by a guy that is considering converting to a 204 when he already has all the "stuff" for a 223.
