Actually, most engagements happen at under 100 m. Typically 150 meters is the maximum... Afghanistan making an important exception.
US Army's standard issue rifle is the M4 Carbine. Most Marine units are swapping to it by end of 2016 too. It has much shorter barrel than old M16(14½" vs. 20") and lower bullet muzzle velocity and thus range too. The M855/M855A1 bullet is likely to rapidly tumble and shrapnel within tissue depending on entering velocity and angle, and that best effective range is at most about 150 meters. Possibly only 100 m. Outside of that, it mostly just makes a hole.
I dont have very reliable or vast data on average or maximum engagement ranges, but it should be telling that US Army and USMC are valuing the compactness of the M4 over the better reach of longer-barreled M-16, and have been reducing the number of .308/7,62 NATO weapons too. The Marine 43-man(3 13-man squads + command fireteam) USMC Infantry Platoon has only 5.56x45 weapons. Only attached support teams may have 7,62 NATO marksmen rifles or M240 machine gun. Same goes for US Army, here is a nice illustration, where the only non-5.56 weapons are the SR-25/M110 marksmen rifles:
That said one can get .223 Rem/5.56x45 cartridges that reliably expand and make horrible wounds up 400 m and beyond... Most of those are just prohibited by Hague convention and dont penetrate steel or protective gear as well, and they're all very expensive.
edit: the UK and Russians actually still have a 7,62 NATO or equivalent caliber rifle in every fire team.