QUOTED FROM THE OFFICER.COM FIREARMS FORUM, LINKED ON THE FIRST PAGE:
So you want to buy an AR-15, huh? Well, you came to the right place.
INTRO
Let me guess, you have seen a few models in your local store, you have seen some ads in the gun rags, and a "guy" you know thinks his rifle is "the best." And now you need some help separating the wheat from the chaff, and have come here to find out what is best for you. Am I close?
I am going to assume you plan to take this into harm's way, or at least want a rifle CAPABLE of being taken onto a "two-way" range. If you just want a recreational shooter, just pick up whatever fits your budget at your local gunshop. If it doesn't work as advertised, no one gets hurt. But for those whose lives depend on meeting evil in this present age with force, and lots of it, keep reading...
The first question you SHOULD ask yourself, is
"what's allowed?" For sworn Law Enforcement Officers employed by a department/agency, this could be as simple as "quality rifle approved by the Range Master." Or, it could be as picky as "Colt or Rock River Arms" (Dallas PD.) So BE SURE you get something that FITS YOUR POLICY, as well as fits your needs. Let's keep going...
Secondly, you should assess your
tactical parameters. Are you a lone deputy out in the sticks? Are you a member of a dedicated SWAT team in an urban setting? This will all play into what you should look into. We'll talk about that later, so let's move along for now...
In addition, we need to consider what options you will be needing, and what accessories you will be running. These can range from slings, to sights, to lights, etc...
Regardless, you will need a rifle that is DEAD NUTS RELIABLE, and made to a degree of quality assurance that you can TRUST it with your life, and the lives of your fellow officers, not to mention the sheep...err, I mean people, under your watch.
Let's focus (for now) on this last point, and discuss what is needed in a fighting rifle.
THE BARREL
The heart of the rifle is the barrel. The barrel is the part of the rifle that directly touches the bullet in its path towards the target, and will be the single most expensive part in your rifle. It is also the one part of an AR-15 rifle that is prone to deteriorating (due to rust, erosion, or being shot-out.) The barrel is what determines the service-life of the rifle, so it should not be the part we scrimp on in order to save a few clams.
So, what kind of barrel SHOULD a fighting rifle have? We have two choices: Stainless Steel (expensive), and Chrome-Lined 4150 steel. 4140 is what most commercial barrels are made of, and is not altogether a horrible choice (as long as it is chrome-lined), but 4150 is still preferable.
H&K, Noveske, and now LWRC are offering Cold Hammer Forged barrels, which are MUCH more durable than standard barrels. These have a service life far surpassing what is currently available, but still need to be chrome-lined. CHF barrels are more expensive and should be regarded as highly desirable.
While SS barrels are VERY accurate, are VERY durable, and are definitely 'en vogue', they are also usually heavier, as they are most commonly made for "match" rifles. In addition to weighing more, they are also more expensive. This can range from moderately more expensive, to outlandishly more expensive! You will also have to be sure the chamber is meant for a fighting rifle, and not a match rifle. We'll talk about that in a minute...
So now that we know what material it should be made of, what about the other aspects? Here is the list of characteristics you should look for:
1- Gov't profile
2- Chrome lined chamber and bore
3- 5.56 NATO chamber
4- 1/7 twist
5- Parkerizing under front sight base
6- M4 feed ramps
7- Magnetic particle tested
If this is what we SHOULD look for, what else could we see? We'll take it spec by spec...
1- Gov't profile
As far as barrel profile goes, other specs would include, "HBAR" (heavy barrel), "Heavy", "MedCon" (medium contour), "SOCOM", or "Bull." While medium contour and Socom mean the same (slightly heavier than Gov't profile, not as heavy as HBAR), they are meant for those who fire their bang-sticks switched to "group therapy", not for semi-auto like most of us will use. All the above nomenclature means the barrel will weigh more than it should, and most likely made the barrel cost less to produce. Other specs would include "A1", "Superlight" or "pencil." These are the smallest barrels, and have a tendency to lose accuracy once they heat up. They still keep to the M4 standards of accuracy, so its not critical. They aren't necessarily bad options, but they do limit your ability to modify your rifle with aftermarket front sight bases later on (barrel profile under the FSB is too skinny for standard aftermarket products.)
In summary: Gov't Profile, SOCOM/MedCon, or Lightweight are all fine options.
2- Chrome lined chamber and bore
Now that we have looked at which profiles we can choose from, let's look at the second spec. Several manufacturers make "Chrome Moly" barrels standard, and offer chrome lining as either an upgrade or they just don't offer it at all. This is unsat. Chrome lining is more than voodoo, it helps in many aspects of your rifle. It allows you to clean your rifle when you can, and doesn't necessitate you clean it NOW. It won't rust, or pit, or for that matter get dinged by a cleaning rod as easily. It has natural lubricity that allows for easier extraction when conditions aren't optimal (dirty chamber, hot round that would otherwise stick, etc...) Chrome also requires no break-in, and it wears at such a slow rate that service life is extended far beyond a standard chrome moly barrel. For the little bit extra it costs up front, it pays off in the end in spades for a fighting rifle. The one downside is a slight loss in accuracy, which will not be noticeable unless you scope it and shoot it over sandbags.
3- 5.56 NATO chamber
The proper chambering for a fighting rifle is 5.56 NATO, not .223 REM. While the cartridges are identical to the naked eye, they ARE nonetheless different. Most importantly, the 5.56 round will not feed reliably in the tighter .223 REM chamber. If you are looking to buy a match rifle, look for .223 REM and buy/load for it specifically. If you want a fighting rifle, you want the looser 5.56 NATO chamber.
Other offerings (usually in stainless steel barrels) can be "Wylde", or "Dutch." They have the characteristics of both chambers, with no drawbacks that I have ever come across. Noveske calls his something else, but it's the same concept as the Wylde. (Wylde/Noveske chambers are designed to enhance accuracy, while Dutch chambers are designed to enhance reliability.)
4- 1/7 twist
The rate of twist for any given barrel is specific to the projectile diameter, length and velocity. For a 55gr 5.56 NATO round (M193), the proper twist is somewhere around 1/10 to 1/12. The original M16A1 came in a 1/12 twist. However, for the heavier 62gr M855 round, 1/12 does not work. The M16A2 went from 1/12 to 1/7 twist, allowing for this round. Truth be told, the 62gr penetrator (SS109) is actually the length of a 69gr bullet, but weighs less because it has a steel core instead of a lead core. This round will tumble wildly in a 1/12 twist barrel. Testing showed this fast twist rate will show premature throat erosion, so commercial barrel makers decided to make their barrels 1/9 twist since they did not need to adhere to the strict military requirement of 1/7. This worked, and the throat erosion seemed to be mitigated. However, with the newest advances in bullet design pushing the envelope to 75gr and 77gr projectiles, 1/9 is not quite getting the job done reliably. Due to tolerances for rifling a barrel, some barrels marked 1/9 can shoot the heavies with no ill effects, but others marked 1/9 throw them down range tumbling wildly. This is not good. A tumbling bullet is not a consistent bullet, and consistency is what produces both accuracy and controlled expansion. So if you want to load your rifle with the most advanced anti-personnel loadings you can, you will want a 1/7 marked barrel. If your duty load is only 55gr or 62gr at most, then 1/9 is probably alright. 1/12 is unsat. You will be stuck with 52-55gr.
While this is not an ammo post, but a rifle post, I will keep it short. A good rule of thumb for the AR-15 is to get the heaviest bullet it will reliably stabilize, as it will penetrate deep enough to reliably hit vital organs, which is your actual target in a gunfight. Ballistic tips do NOT reliably penetrate deep enough to hit these targets when conditions are "imperfect." Stick with BTHP (OTM) ammo, and get the heavies. Our duty load is the Winchester Ranger 69gr BTHP (which works well in 1/9 twist barrels), but my personal choice is 75gr OTM (which I carry in my 1/7 twist LWRC and CMMG.)
5- Parkerizing under front sight base
This is one of those features that is an indicator of quality, moreso than it being an actual feature. However, when I switched out my standard FSB on my Bushmaster rifle to a railed gas block, it exposed some of the white metal that was not parkerized. While it did not rust
on me, it was annoying. I later switched back to a standard FSB, but I still KNOW it's "naked" under there...

Just to reiterate, this is not a critical issue.