The ordnance department may have done the math and decided after the report that a 120 grain .224 bullet would have been way to long and heavy for caliber. With the powders of the day it may have been very difficult to stabilize the projectile properly.

The Greenhill Formula suggests the longest .224 bullet that can be stabilzed with a 1 in 7 twist with velocity 2800 fps or greater is 1.290 inches.  I could not find any .224 bullets that weigh 100 plus grains that are commercially available. ( I don't think anyone manufactures a bullet this heavy for .224) The avg length of the 90 grain bullets available is about 1.2 inches.  The avg length increase of .224 bullets is about 80 to 90 thou per 10 grains. So a 120grain .224 bullet would be about 1.53 inches long. A 1 in 6 rate of twist would likely be required to stabilize a 120 grain .224 bullet that is 1.5 inches long.

REFERENCE MATERIAL:
The Greenhill Formula

The Greenhill formula is an empirical equation that does a good job of establishing the barrel twist necessary so that a bullet of a given length will be adequately stabilized.

T= The twist required (number of inches for one revolution)
D= The bullet diameter (in inches)
R= The bullet length to diameter ratio (length divided by diameter)
K= Velocity constant
     Use 150  for velocities between 1500 and 2800 fps
     Use 180 for velocities above 2800 fps

Rate of Twist = K x (D/R)
                  

Conversely, to find out what length bullet will be stabilized in a given twist, use:

Bullet Length = K x (D x D/T)

Note that it is bullet length, not weight that is important.

Max Length bullet calc. using above formula.

Caliber      Bullet Diam.      V. Constant      Length for 1 in 6
223 Rem    0.224                180                         1.51

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