The Statistics of the AR-15 (as per the BFI XM-15 manual) are as follows:
Maximum effective range: 550 meters (individual/point targets) 800 meters (area targets)
Maximum range: 3534 meters
I don't have it handy but these numbers agree with my recollection of the statistics for the M-16A2 in the Guidebook for Marines.
I have always considered the M4/M16 to be a short range battle rifle (short/med range at best) and given the ballistic properties of the 5.56 round which are lost with distance, I question the validity of these numbers.
The battle rifle's "effectiveness" IMO is the rifle's ability to take down a man from a distance. assuming that ammo doesn't varry (XM-193, M855, etc) the main variables that I see in the the M16 & M4 are barrel length (affecting accuracy as well as velocity) and optics. Granted, a rifle that relies on fragmentation for effectiveness won't have a max effective range (IMO) of 450 meters with optics when the round won't take a man down at 300 meters with iron sights.
Bearing all that in mind I would ask what you consider the maximum effective range ( in terms of accuracy AND velocity) of the following:
1. M4 with iron sights
2. M4 with magnification optics
3. M16 A1 with iron sights*
4. M16 (A2, A3, A4) with iron sights
5. M16 (A2, A3, A4) with magnification optics
* If you would like to include 3a. an A1 with magnification optics then knock yourself out.