There are many factors to the "best" AR for target shooting, and I have these, in order:

• Match Grade:  Some experts say that a "match grade" AR has closer tolerances than a "Mil Spec" AR.
  
  
• Accurate barrel:  A heavy bull barrel will minimize harmonics.
  
  
• Light trigger:  A good target trigger should be 5 pound or less, crisp trigger minimizes flyers.  Military spec is built for durability, not necessarily high accuracy (close enough for government work).  For example, the M-14 AR triggers are very hard and durable, but you can tweak the spring to make is less poundage, or better still, drop-in a 3 pound AR competition trigger, like the AR Gold from American Trigger Corporation.  I hear that the "American Trigger Company" makes an "AR Gold" 3 pound drop-in trigger for competition shooting, installed in minutes and ready to shoot.  You can also mess with the springs in the Mil Spec triggers for a lighter feel.  (WARNING:  This AR trigger DOES NOT work on the Colt AR-15's if it has a "sear block".)
  
  
• Good Sights:  High quality battle sights are a must, as are a good telescopic sight.  See my notes on mounting a rifle sight on an AR-15 or M-4.

The best AR target rifle

The best rifle to fit my needs was a Colt AR-15 heavy barrel match target rifle.  The AR-15 MT 6700 Colt (Match Target) is the perfect assault rifle for home protection, especially since you can quickly swap-out the bolt carrier unity for a .22, allowing you to kill a home intruder at up to a third of a mile away.

AR-15 Match Target rifle fitted with .22 caliber magazine

It's also called the "Blue label box" with the heavy barrel (8 1/2 lbs) for high accuracy at distances greater than 500 yards.

The salient characteristics of the AR-15 are:

• Looks like an M-16:  While it might be "cool" to have a gun that looks like an M-16, it's not cool at all if your opponent has a real M-16.
  
  
• Very versatile:  The AR-15 is super easy to clean and you can swap-out toe .223 bolt with a .22 bolt in less than 60 seconds (see below).
  
  
•  Jerky trigger pull:  I found the trigger to he jerky and unsmooth, with a high force pull to discharge, something I can tweak to make smoother.  The trigger on an AR-15 is designed like a shotgun, with high trigger force, and it make an “expert smooth pull” is difficult.  See this AR-15 forum discussion for AR-15 trigger smoothing, and here are compete instructions for AR-15 trigger smoothing.

BEWARE:  Too much trigger manipulation can be dangerous.  You might cross the line between a hair trigger and a machine gun, and making an AR-15 fully automatic (into an M-16) is a felony.

AR-15 shooting yardage

NRA High power rifle competitions are done with AR-15’s at 200, 300 and 600 yards.  These NRA rifle competitions are shooting with open sights - no scopes.  Scopes and red dot sights are for grandma.

At 800 yards and beyond, the AR-15 does not have the muzzle velocity for a clean kill, and it’s like lobbing a cannonball. Military distances for marksmanship competition
Military qualification as expert with the rifle (M-16/M-4) requires 35 to 40 hits out of 40 rounds (on various sized silhouettes, simulating targets at different ranges.

The military rifle targets are placed at 25 meters away, with silhouettes simulating targets at 50 meters, 100 meters, 150 meters, 200 meters, 250 meters and 300 meters. Ten shots are fired each from these positions: