The M1 has accuracy and dependability, the sights are far superior to anything any other country had to offer, all other designs however good, did not allow the shooter to adjust to the ability the M1 had, the 1903 A3 had very adjustable sights as well, the US, decided the shooter needed to be able to adjust the sights, where other countries just gave elevation adjustment with crude ladder type rear sights. Most of these other rifles were sighted in at the factory and the troops just used what they were given, The US, made a point of making soldiers RIFLEMEN. I think this makes the biggest difference, if you gave a US rifleman a foreign rifle and asked him to shoot it, it would print where it did, and then he would ask " how do I make shoot where I want?" The Mk4 Smle, did have decently adjustable sights, not really all that accurate, but sturdy, and usable.
And the .303 is not a bad round, it hits as hard as any other round used, 173 grn bullet at 30/06 velocity. That's not hay.