If fragmentation isn't an issue then go to a heavier bullet. A 1 in 9 twist is good for up to 62 grain bullets. A friend of mine uses 69 grain bullets to good effect but they won't print in my rifle. Most match shooters use heavier longer bullets. Be careful though, I have seen some loads that won't fit in a magazine and have to be loaded one at a time.

The rifle will probably be more accurate than the shooter in most circumstances, and you will find that you are the limiting factor to accuracy at a distance.

 

800 meters in soft tissue, no body armor.

If you could hit a man sized target that far out.

 

I don't know the answers regarding the M4, but as you doubtless know, the M16 version (with a 20" barrel) is routinely used with high accuracy and precision at Camp Perry out to 600 yards.

While this is obviously well beyond the round's capability in practical defensive and offensive use (I'd guess a 55-75gr bullet is travelling maybe only a few hundred feet per second at 600 yards), the capability for hitting the target may still be quite high, given sufficient holdover (600 yards being well outside the limits of "maximum point blank range"), though its effectiveness would be poor.

I believe most folks consider it effective as an anti-personnel round inside about 150 yards, or thereabouts.

Just FWIW, I believe that the popular current trend toward barrel-shortening in military rifles is ill-advised, given the dependence of velocity upon barrel length and the need for higher velocity with lightweight bullets for sake of terminal ballistic effectiveness, especially for the "weaker" cartridges like the 5.56x45.
I do understand the desire for rifle portability and "handiness", but I consider any barrel length less than 16" with an AR15-platform .223/5.56 rifle to be an unacceptable sacrifice (in terms of ballistic effectiveness) for the degree of proportionately-minimal benefit (in terms of portability) obtained.

Best.

 

one of the key things to remember, as much as we talk about long range accuracy and lethality,is that over 90% of all combat engagements with the rifle take place at 100 yards and less , many within ranges normally considered to be pistol engagment distances- a compact carbine/m-4 was originally designed for these close quarter applications and to issue to troops that were not primary war fighters- cooks,MP's and mechanics etc.(although in the current middle east scenario anyone can find themselves a key player in the fight)who would not be engaged in long range firefights. It was (and still is) better to have a short rifle "carbine" like the m-4 as opposed to the pistols and SMG's issued to such troops in years past. The problem is the primary fighting infantry became interested in the shorter lighter"sexier" m-4 and now primary infantry have it,which was NOT the original intention of its fielding.

 

Note that the 75 and 77gr open tip match (OTM) bullets increase both accuracy as well as effective range because they will fragment at significantly lower velocities than will either M193 or M855 ball ammo.

The 68gr OTMs (like the Black Hills load) is somewhere in the middle, but incredibly accurate out of just about any AR with a 1-9 twist or better.