Part of the difficulty is definition - a battle rifle is a .308 equivalent, likely magazine fed, and often could fire full auto, although not very well. That's why civilian clones may not be actual issue weapons.
Another is that most accuracy testing, like done for the US military, uses standard milspec ammo. And that standard is 2MOA shot through a milspec barrel. A good handload could reduce the group size to 1MOA.
Another issue is that most military rifles aren't built to high accuracy standards. 2MOA is the battle standard for many countries. That's a 10" circle at 500 yards, the typical maximum the average soldier might attempt - if they had an easily visible target that remained stationary, and confidence in their skill. Ten inches of an 18" to 24" hit zone is sufficient for combat. Woodchucks, prairie dogs, or antelope would be much more difficult.
Since soldiers dont have a need for more accuracy in battle rifles, makers don't spend the money on precision barrels. In the day, as rifles were assembled, if one test fired with great accuracy, it was held aside for precision use. That likely was one in a thousand, and it was cheaper to do that than build special ones.
Other issues get involved - wood stock guns with barrel band swivels aren't free floated, so sling tension will make them shift the point of impact. Rifling buttons were standard profile, and the process part of mass production, not precision work. Complex receiver designs with pressed or pinned assemblies meant you couldn't adjust the headspace as much as just get it in the tolerance window.
Compare that to the AR15, which has run off the battle rifle in precision competition, simply because the design supports a lot of improvement, without expensive gunsmith level work. AR15 are dominant for very good reasons - ergonomics, straight line action, little recoil, easy to free float, barrel installation done without a press or expensive tools. Optics mount over the bolt carrier, controls work properly from the trigger hand without shifting the grip, and the off hand complements it.
Argue about where to put the piston all we want, what most don't see is that most of the other Stoner principles are included in new combat rifle designs. Operating rods, open, stressed receivers, and wood stocks are not coming back. They are inefficient, bulky, and classed as curios and relics of a bygone age.