I think Hatcher's book is the reference that stated that there is no substantial difference in accuracy between the 2 and 4 grooved barrels. But, I'm sure the testing was done with govt. ammo (bullet weight and powder). Of course, it's faster to produce a 2 groove over a 4 groove barrel, a welcomed production saver during wartime.

Remember, the WWII sights are battle set for approx. 600 yards. Longer bullets are more stable in flight than shorter bullets, and so, are more accurate at longer ranges. But so many factors influence accuracy including type of bullet (hollow point, spiral point, flat point, boat-tail vs. flat-tale).

Keep in mind that rifles (commercial or USGI) designed to hit the mark at 1 mile + ranges are surely 4 groovers. So, the longer the distance, the 4 groover will have the advantage over the 2 groover, eventually.