The barrel probably has the most affect on the AR rifle’s accuracy & performance.  There are a handful of attributes to choose from when selecting a barrel, and you’ll want to to choose the right ones that help meet the needs of your desired rifle.  Click this post to access more info below:

With each variable below… I’ll also explain what aspect of your barrel it affects.  Before going through this list… keep in mind what type of AR you’re going to build.  A short, tactical close range AR?  Maybe a long range hunting rifle, or varmit eliminator?  Multi-purpose zombie killer?  Your overall goal should be what drives your decision making when picking barrel attributes.  Here we go:

Length – Barrel length affects accuracy and muzzle velocity of your bullet.  The longer the the barrel, the more accurate your rifle will be at long range.  Your projectile will also travel a further distance in the barrel with extremely high pressure behind it allowing it to accelerate faster.  On the flipside, a shorter barrel is lighter, and easier to maneuver in confined areas.  If you’re clearing a home or in an urban setting defending your life / family… you might not want to be swinging around a heavy and long barrel.  With 16″ being the most common, I saw AR-15 barrel lengths range from 7″ to 24″ with various stops in-between.  The barrel length will affect your Hand-Guard lenth, and gas tube length, so be conscious of that when ordering additional parts.

Twist – The  rifling inside your barrel act on the bullet to cause a spinning gyroscopic affect on it.  The barrel twist is expressed in a ratio, like 1:7 or 1:9.  The ratio refers to the number of inches it takes for the bullet to make a full 360 degree turn.  So a 1 turn in 7 inches twist spins the bullet faster than 1 turn in 9 inches.  The faster you spin your bullet, the more stable it should be when traveling through the air… but there’s a catch, you can OVER spin light bullets causing inaccuracy and drift (bad).  So keeping things simple,  you choose the barrel twist based on the weight of the bullet you plan to shoot.  Bullets from 35 to 50 grains in weight  should be used with a twist of 1:14 – 1:12.  1:9 (most common) and 1:10 are suitable for 45 to about 70 grains.  The most common weighted bullet I’ve seen is 55 grain.  The military steel core ammo I saw for combat was 62 grain.  1:9 twist seems to be the winner as ‘most likely compatible’.  For 69 to 90 grain bullets, a twist rate of 1:8 or 1:7 should be used.  Not sure what type of ammo you’ll be using?  Take a look online at some ammo stores and maybe you’ll see some common weights and pricing like I did.

Chrome Lining – There is much debate over this topic, and apparently a lot of myths… so I’ll keep this one simple.  Chrome lined barrels offer easier cleaning, higher resistance to corrosion, and higher resistance to super hot temperatures experienced during rapid fire.  The US military used chrome lined barrels, but it isn’t necessary for civilian use at the range.  It is said that a chrome lined barrel is less accurate, but the CDT blog stated the difference being about 1/4″ at 100 yards.  Your budget may play more of a role in this decision, as chrome lined barrels are significantly more expensive than non-chrome lined ones.  I may stick with the basic barrel for my build, but I need to check out the pricing differences first.

Twist Source: http://cheaperthandirt.com/blog/?p=2625
Chroming Source: http://cheaperthandirt.com/blog/?p=2678