Selecting a barrel can be tough. They are the single biggest part that affects the accuracy of your rifle. With a great barrel to start with, you can use cheap or second hand parts for everything else and still have an extremely accurate and reliable rifle. But there are so many choices. Even picking a length can be tough, let alone contour, gas system length, the alloy it is made out of, the process used, twist, type of rifling, fluting etc etc. Some of the choices of these are easy, some are much more complicated.
Note: By “all purpose”, I mean you might use it for room combat, you want to be able to hit out to 300, you want to be able to get in and out of a vehicle without too much difficulty, you will run a variety of ammo through it and you want it to last a reasonable amount of rounds. If you want the best value for an all purpose barrel, try Rainier select or BCM in the 16in length. For best price and quality for a premium long range barrel - Lilja 20in. They even stock them so with luck there is no wait.
However, if you have the time and interest and you want to know more about selecting a barrel, just pick a category below...
Is there a perfect Barrel Length?
Yes and No. There is a reason that they make so many different lengths- there are pro's and cons to every last inch. However at the extremes some of the pro's disappear - Depending on ammo, there are diminishing returns if you buy longer than 20 inches for 5.56. Additionally, you should think carefully before going below 10.5 inches as the round needs that long to stabilize. Most manufacturers of suppressor/silencers list 10.5 as their minimum barrel length to put the can on without voiding the warranty.
With every inch of difference, there is a resulting change in velocity, which affects both accuracy at range and terminal ballistics, or in other words, if you want to kill beyond 100m, or 200m every lost inch reduces your ability to hit or kill what you are aiming at. Though it is important to note that the difference becomes less per inch with length - the improvement in velocity for a 2 inch increase from 8.5 inches to 10.5 inches is much more than 20 to 22 inches.
For some shooters, a shorter barrel offers no gain. However for most, being easier to maneuver and conceal is worth some loss of velocity. Being able to get closer to cover when you need it, to transition to a pistol and not worry about getting a muzzle full of dirt or being able to rapidly enter a room or get out of a vehicle without snagging the barrel can all be more important that 100ft/sec depending on the situation.
The choice of length is up to you but my advice on it is this: While there is something truly awesome about the power you can get out of a tiny package with an AR with a 7.5inch barrel, you should always pick the longest barrel that is usable for your intended task. Remember that other than increased concealablility, reduced weight and the factors mentioned above, there are no other gains from a shorter barrel (other than cool factor of course, which does have some importance).
Finally before buying a barrel there is another factor to consider that sadly all to important for us to adhere to: You probably already know about it but if not, I hate to be the one to tell you about it - The National Firearms Act. Some states have additional legislation to consider also, but generally speaking you can have a barrel shorter than 16", but it either needs some kind of muzzle device permanently installed to it to bring the overall length to 16", or you need to get a stamp and go through some months of paperwork plus pay $200 to have the shorter barrel.
Gas System Length
What is the best Gas system Length?
Thankfully, this is an easy choice. There are 4 lengths available - in order of shortest to longest they are pistol, carbine, mid length and rifle length. Just get the longest your barrel will allow. There is no benefit to a shorter gas system. Shorter means the gas that hits the carrier key is less burned, hotter and expanding faster. Recoil is sharper and part wear more substantial. You can have an extremely reliable rifle with a 16in barrel and a rifle length gas system. At any barrel length beyond that, stick with a rifle length gas sstem. For 14.5in, use a mid length. Below that, most manufacturers limit your choice to carbine length, but there are a few with a pistol length and a 7.5in barrel.
At first I was skeptical about the differences, but I have side by side shot the exact same upper configurations with the only difference being gas system length and I was amazed at the change in how it felt. Shooting mid length versus a carbine length or a rifle length has a very noticable difference in felt recoil and the ease with which it is managed. I have seen bolt after bolt fail on the line (mostly torn locking lugs after about 8-12k rounds) using a carbine length system, when rifle length systems just kept running (for well over 40k and counting). The bolts all come from the same place, only the uppers and gas systems are different. The cool factor of a sub 10.5in barrel is pretty high to be sure, but I have seen 2 pistol length gas tubes fail catastrophically under 2000 rounds. I am not saying don't get a badass street sweeper style AR-15 SBR, just know you are on the border between reliable and unreliable so be careful and always get the longest gas system you can.
What is the best Alloy for barrels?
There are 4 choices regularly offered: 4140 CMV, 4150 CMV, 410 Stainless and 416 Stainless. There may be others, but more likely it is a different treatment process of one of the four listed. They all have their pro's and cons, but milspec is 4150. If is chrome lined, you will have a durable barrel that may or may not also be accurate. 4140 is a lesser, and cheaper alloy, but it still takes a good shooter to outshoot even a cheap barrel. The top match barrels are generally made from 416 Stainless. Some may be made from 410, but it is a harder alloy to machine, which in the case of barrels is not always a good thing.
The important thing is to find a good manufacturer and trust they know the best way to get most out of their blanks, whatever they might be made of. It is not like all alloys of the same SAE designation are the same anyway, there are differences in batches and between steel mills and then treatment as a blank.
Barrel Manufacturing process
In all the world of shooting, there is perhaps no bigger argument over any single topic than about how the greatest barrels are made.
For all my research on the topic, I have learned many things, but perhaps the biggest thing is this:
What is generally more important than the actual process used to make a barrel, is how well the process is done. As with all things shooting, consistency is accuracy. Simply having the bore end up the exact diameter desired at the end of the manufacture process requires a lot of knowledge and experience. It is a mix of both art and science
The drilling, rifling, finishing and lapping all change the bore diameter and doing any one done inconsistently for the caliber and alloy being produced will make a less accurate barrel.
For most consumers, it is easier and safer to forget about all other factors and just find a good manufacturer. You don’t need to look far as I have listed some for you.
Cut rifled, broach rifled, button rifled, hammer forged, cryo treated, heat treated, double stress relieved, roll threaded, lathe threaded and so on are all listed as features that make one barrel better than another. But they can all be awesome if done well and all be inaccurate if done poorly. What differentiates the best barrel makers from the lesser makers is how good they are at doing the process and how exacting their standards are. When you pay for a top barrel, you are paying for expertise, experience, the equipment used to make the barrel and the stock steel the barrel is made of. But more than that, you are also paying for the metal in the scrap pile that didn't make it as a barrel. Metal that might have made it out of another shop as an acceptable part.
However, there are still a few pro’s and con’s unique to each process, however well they are done and if you are interested enough to keep reading, you can rank the 3 most common ways:
Cut rifled, if done well, is the most accurate. It stresses the barrel the least as each rifling groove is individually cut
Button rifled, is much faster and easier, but if done well it can still be extremely accurate
Hammer forged, is very fast, required expensive equipment, stresses the steel the most, but also work hardens it. It makes for the least accurate, but most durable barrels
What is the best M4 / AR-15 Chamber?
This is again a somewhat easy choice to make. If you plan on running only (or mostly) military ammo through your rifle, just get a USGI 5.56 NATO chamber. Many people want to be able to run anything, which they can with a NATO chamber, but for more all around accuracy across a variety of 5.56/.223 ammo, the .223 Wylde chamber is the easy choice (or Noveske Match Mod O). Both were designed to bridge the gap between .223 Rem and 5.56 NATO and they do it well. It is almost exclusively the only chamber used by the top barrel manufacturers. Most chambers will accept most rounds of the 5.56/.223 family with no problem, but for the ultimate in accuracy and reliability you have to perfectly match the round to the chamber. Additionally, you should not run 5.56 NATO ammo in a .223 Remington chamber. Many still do, but the closer rifling and steeper leade creates pressures that exceed specs. Can you get away with it? Yes, maybe for a while even but that is not a failure I want to be around for when it happens.
There are some compelling articles, like this one that point out the fact that while SAAMI specs/dimensions are different between 5.56 and .223 chambers, there are only a few manufacturers that can ream a chamber to the kind of tolerances that would appreciate those differences. Even from the same brand, you might get 3 chambers listed as .223 Remington, but in actual fact the variance between them is bigger than the difference between .223 and 5.56.
However, to me that is further proof you should not tempt fate with 5.56 in a .223 Rem chamber. It just increases the chance of a kaboom if you are already over pressure and then they reamed the chamber in a such a way to further increase that pressure.
Choosing Barrel / Rifling Twist
Barrel twist is measured as a ratio like 1:7, 1:8, 1:69.7 etc. That means 1 revolution of the round (due to rifling) takes the 7 inches, 8 inches or 69.7 inches etc. So the bigger the second number, the slower the rate of twist. There is so much BS on forums about twist it is truly staggering. And yet you can call any quality barrel manufacturer and get a solid answer. Or any decent ammo manufacturer.
Generally, by selecting a chamber, the barrel maker already perfectly pairs your twist rate (though there is some whack stuff out there), which further confounds me as to why there is such confusion on this subject. For example. BCM makes a great milspec type barrel. Chambered in 5.56 NATO, designed for the M885, 62gr green tip ammo. It comes in 1:7 twist. Most .223 wylde chambers are paired with 1:8 - All round performance matched with all round performance And of course, .223 Rem chambers usually come with 1:9/1:12 twist. Shorter, lighter round, less twist.
I think part of the reason for the confusion is the very real results that can come from pushing the twist to the extremes and more importantly the difference in performance you get depending on what way you push the limits. Just as the 5.56 Chamber runs any any .223 variant with no issue and a limited loss of accuracy, 1:7 twist runs any shorter, lighter round with no issue and a small loss of accuracy for each weight you step down. However, going the other way can produce horrible results. If you get a 1:12 twist barrel for varmint hunting and try and run some 90gr sierra through it, your group size will baffle you. While it is possible to over twist a round, it is much easier to under twist it. More round length means a faster twist is needed.
Most simply put, more twist means more stability. So more twist is better, so long as you don't take it to the point it starts to deform or destroy the bullet.
Does fluting work?
In a word? Yes it does. It gives a greater surface area for cooling, reduced weight and minimal loss of rigidly (if done correctly). Rigidity is actually significantly higher in a fluted barrel, when compared to a barrel of the exact same weight, however it is a little lower compared to the exact same barrel that has not been fluted. And more rigidity/stiffness from your barrel equals more accuracy.
Fluting and barrel stress
There is a lot of science and experience on this, and it does not all agree. Some say fluting stresses the barrel and it should be stress relieved after it. Others say removing material from a barrel causes no stress. It will take me a lot more learning and experimenting before I am comfortable saying who is right. In the meantime, I will defer to Dan Lilja's words on fluting and stress. He says they flute 50 barrels a month with no stress relief and have never had a reported problem. Despite that, I still don't advise getting your barrel fluted just anywhere, or by just anyone. While it is a simple process, it is very possible to screw up and I have seen some horrible gun work come out of shops.
What fluting is best?
The choice used to be limited to the number of flutes, but now there is dimpling, spirals and all kinds of other types available. They look cool and they do still work. But they don't work as well as straight flutes.