Does anyone use IMR 3031 for .223?
The Army does.
The Army used a Mann Accurach Device to test their ammo. The one for the 5.56 is a 1:7 twist barrel on a Remington Action. Super tight chamber, and 18 inches long.
To test the device they load 25 grns of 3031 in a Remington Case, match primers and 52 grn match bullets.
The grouping averages about .18