It rather depends on several factors. My background is the Army Small Cal Wpns Lab and then Test Director Small Arms at Aberdeen Proving Ground. I have conducted multiple tests of 5000, 10,000 and 12,000 rounds on shoulder fired weapons and evaluated the systems for the gov't as a Certified Test Director. I conducted the testing on the M16A1E1 which was adopted as the M16A2 and shot up 244,000 rounds in the effort up to 800 meters. Thus the following is from actual experience and not gleaned from reading blogs, gun rags etc.
1. How accurate do you want it to be?
2. How long do you want it to last?
3. What are you going to use it for? If it is for conventional highpower competition I would get a 20" barrel. If it is to be for varmint hunting I would go with a 24" barrel
Shorty barrels (yes I have a 16 1/2" AR) looks sexy but it depends on what you plan to do with it.
For instance the other week I got called to come shoot a coyote in a neighbors pool. I got there and found she had just got out of the pool exhausted from swimming all night and was laying at top of steps. I tagged her dead center right side with a M855 round.
She got up and dove back in pool ! ! ! ! Had a 4" exit hole on back side. Round took a downward angle after entry. Range was only 18 yards and she got up. She only weighed about 45 pounds.
The M193 and M855 rounds (source on this is the Army Wound Ballistics Lab) has highest lethality in first 45 yards. Does well up to 95 yards giving break ups into multiple pieces.
After 95 yards the rounds break in half at cannelure giving two pieces for the M193 and three pieces for the M855 up to 200 yards.
After 200 yards the rounds do not come apart, deform a bit, tumble and that is that. I saw a news clip in Iraq where this raghead was apparently coaxed into going out in the street with his RPG and shooting at Marines down the block. He went out and assumed a kneeling position and before he could touch it off he took two rounds of M855 and it just shook him a little. He didn't realize he was shot and turned to look at press guy with a quisical look and the third round was a head shot which got his attention.
Accuracy: Got acceptance accuracy is 4.5" at 100 yards. Barrel is considered gone at 7.2". Neither of which is acceptable for my personal rifles. I now have 5 ARs and have owned five others. I dumped the originals when the floating barrels came out. Just having a floating barrel will help accuracy tremendously.
I have two Daniel Defense 4 rail uppers 14" long and feel they are excellent as I want to mount a bipod at the front of the DD. One is 16 1/2" barrel with flash suppresor and other is 20". Both from superiorbarrels.com The are more expensive than most on market but they will hold up far longer. To my knowledge no one has shot one out yet.
To me a shot out barrel is one that shoots over 1" for a target rifle and 2" for a hunting rifle. For a survival rifle I would split the difference and say acceptance would be 1.5" and rejection 2.5" at 100 yards.
AR Match Grade barrels are normally replaced at 2000 rounds by civilians. THe Army Team replaces theirs every 700 rounds as heavy bullets take them out quickly.
I also have a match AR with superior barrel set up in A2 configuration so I have three of them. The other is a RRA 24" varmint I have never fired. Guess I should sell it as I have had it three years and not shot it. Lastly is a pre ban Colt A2 with full match rework, floating barrel, target sights sets.
All five have match grade 2 stage triggers. 3 20" barrels, 1 24" and 1 16.5".
The DD 4 rails have scopes.
Chrome lined barrels by and large don't shoot as accurately as unchromed barrels but occasionally you will fine one that shoots quite well. If so keep it. A good chrome barrel will hold up about 3600 rounds and then starts to open rapidly with SS109 ammo.
Barrel life on unchromed barrels is dependent on your loads. I have seen wrong propellant used take out three chromed barrels in 6000 rounds and accuracy was about 8X12 feet at 600 meters fired from bench rest. With correct propellant they went 12,000 rounds and were right at 7.2". This was with genuine SS109 FN made.
Shooting combloc steel jackets will wipe a barrel out quickly. I borescoped one AR barrel with a recorded 5000 rounds on it and the rifling was worn down even with the grooves on the front half of the barrel.
You need to redesign the brass deflector (bump to rear of gas port) to make the front of it at right angle to the ejection port. This will direct your brass ejection to between 1:00 and 2:00. Several have been killed and also seriously wounded because a guy next to them got a piece of hot brass down his neck and started the Watusi 2 step and lost muzzle control and accidently shot the guy next to him. Many are walking around with permanent scars on their necks from hot brass. The M1 and M14 are designed to eject brass between 1:00 and 2:00.
I have made deflectors several ways. There is a metal one that clamps on flat tops that works well. Anolther one is molded epoxy putty. The others is 7/8" square Velcro peel and stick packs from Home Depot. Degrease bump and stick the hook side on the bump. Cut a piece of Armaflex insulation in a small triangle and stick the fuzzy one on it. Brass will drop down very closely to your side and won't leave your firing point if you do everything right.
This problem was documented in the Aberdeen Report as a "catastrophic occasional deficiency of the system". Scroll down to 1983 at
http://www.thegunzone.com/556dw-8.html
Something to consider if you cause someone to get shot you are going to wind up in court spendings over 100 K to stay out of jail. Then as well you could be the one that got shot because you didn't fix your own ejection problem. At the very least you are gonna get sued to h#%L and gone and most likely lose about everything. I have all my rifles fixed so I don't get brass sent directly at shooter on next firing point. As well these systems produce fired cases that are not deformed and look as if they were fired in bolt rifles.
Your sight acquisition is going to be iffy past age 30. No one on Army rilfe team now is over 30 unless they are coaches. With the close spaced iron sights of the shorty it is even worse. Don't believe it, go out and shoot bench rest at 600 yards with both and see which one is worse. We shot the A2 versions to 800 meters with SS109 and they did decently for a military rifle with iron sights which was about 40" at 800 meters.
With a good scope I estimate groups would have been around 20" in the hands of a good shooter and no wind.
Bottom line is if you want to shoot accurately you need to go one direction. If you want to shoot at tin cans and hose out thousands of rounds of cheap ammo then anything would do. If you want to depend on it for survival applications then I would not.
95% of the AR parts floating around out there will not meet the gov't drawings for the parts. Thus the saying "if it looks like a duck, walks like a duck and quacks like a duck it must be a duck" is not applicable and the system you buy will in all probability not deliver the same performance/endurance standards the military expects.
Regardless of what you get you need to address the brass ejection issue. The life you save may be your own or a loved one.
