No, it isn't dependent on how you fire it. It's just the nature of the weapon and the caliber. Max effective range is usually about 300-400m. Troops overseas can literally stand in the open while insurgents' bullets land at their feet because AKs just can't reach them. An AR15 is by no means a pea shooter against an unarmored target like a zombie.
I think not, the 5.56x45 is a heavily criticized round by troops all over the world. Many tests have shown that the 5.56x45 is accurate yes, but fails to inflict proper damage beyond 200 meter. Testings done has shown that a 5.56 has very bad fragmenting when it hits a target, also when the bullet impacts beyond 100 meter there's a high chance the bullet wont "Yawn" when hitting a target
Also insurgents are poorly to not trained persons. It is not the rifle alone, it is the person who has trained with the rifle, who has adjusted the sights to his specifications and so on.
But be damned sure that NATO soldiers are scared off the insurgents armed with lee enfields or mosin nagants. Many casualties has fallen to these WW1/2 era bolt action rifles.
Why do you think Troops are begging for the new 6.8x45 SPC?