You're asking what is "effective" without defining what you consider effecitve.
M855 will reliably fragment out to about 50 yards and M193 out to about 100 yards. Using Winchester PP, you'll get expansion out to 300-350 yards or so. (Note that these are estimates; environmental conditions have an effect on ballistics, so nothing is absolute.) Other types of bullets will have different performance envelopes and damage profiles. You can't make accurate generalizations that apply to these different types of ammo.
So, what is "effective"? In the minds of most experienced folks, "combat effectiveness" means a weapons system that creates enough damage to take the enemy out of the fight. If your definition is different, so will your answers be.
Standard military FMJ won't damage much tissue if it's moving too slowly to fragment, or if the bullet hits in too thin of an area. But, *IF* you can hit someone's Central Nervous System [CNS] (i.e., head/spinal column), you will almost certainly take them out of the fight. But the CNS is a fairly small target. The spinal cord is an inch wide, and protected by big bones. The brain is a bigger target, but is armored by a rounded skull.
If you manage to get a head or spinal shot at ranges out to 500m with military FMJ, your shot will be "effective." If you don't, you may cause a wound that kills your enemy eventually, but one that probably won't prevent them from continuing their assault in the short-term. In other words, a non-CNS shot with military FMJ beyond fragmentation range is usually NOT "effective" by our definition.
The problem is that weapons manufacturers use "accurate enough to hit a target at x meters" to define "effective." Under that definition, an M4 is effective to 5-600m. But they make no mention of damage or wound profiles at those ranges. This shouldn't surprise you: it's called MARKETING.
In my opinion, the M4 is a specialized, CQB-type weapon. In that role, it is excellent. It is NOT a general-purpose rifle, though. For that, use a 20" rifle.
The whole Black Hawk Down experience should have taught us this.
-Troy