AR15 Barrel Options & Definitions
8. The Barrel of the AR15 is one aspect of the entire platform that someone can obsess about for quite some time. And for good reason since the barrel is the final word on accuracy and reliability, and sometimes cost, of the entire AR15 build. With so many options like length, gas system, caliber, twist rates, gas system lengths, and each variable having pros and cons; the barrel is one area of the rifle that should not be neglected! Proper selection of the AR15 barrel can be a book onto its’ self.
One must weigh the pros and cons about all aspects of the barrel when making a decision. For example talking about supreme accuracy with pistol length barrels may be a moot point. Accepting a slight loss of supreme accuracy, but still maintaining combat accuracy and reliability may be a good balance. The intended use of the AR15 should really dictate the type of barrel used.
A. AR15 Barrel Profile
Government Profile is thicker than the original Light Weight A1 style barrel. The M16 and M16A1 profiles were thin lightweight barrels with .625″ diameter at the gas block. The Government profile A2 barrel was made with a greater thickness from the gas block to the muzzle to resist flexing and to allow a longer period of sustained fire without overheating. The rest of the barrel was maintained at the original thickness to allow for the the M203 grenade launcher to be attached.
Lightweight, AKA Pencil, is the original profile barrel. It is .625″ diameter at the gas block.
Proprietary & Special Purpose
B. AR15 Barrel Materials vary.
Stainless Steel barrels are generally considered to be the most accurate and are often preferred by bench rest and competitive shooters. They are also a great choice for hunters as there is less chance of corrosion due to weather.
Chrome Moly Vanadium (CMV) are steel barrels and can be considered the starting point for any AR15 barrel. The name can be misleading, and these are not chrome lined barrels. CMV barrels are the most affordable barrels on the market, but require the most maintenance to prevent rust or corrosion. If you’re really good about cleaning your barrels religiously, this might be the right choice for you.
Chrome Lined barrels are considered to be the most durable of the AR15 barrels. This is because the additional chrome lining adds self lubrication and prevents wear. The self lubrication also aids in extraction. However the trade-off is a slight loss of accuracy. Chrome lined barrels are great for tactical, home defense, or hunters; but might not be the preferred choice of a bench rest shooter looking for supreme accuracy. Just how much accuracy is lost? It is generally accepted to be about .5 MOA when compared to stainless steel barrels.
C. Rifle Cutting (Rifling)
Rifle cutting, or rifling, can be button/cut or forged. Cold Hammer Forged (CHM) barrels are considered to be more dense than cut rifled barrels. This is because of the forging process hammering the lands and grooves into the barrel, no material is removed. Whereas with a buttoned or cut rifle barrel, the lands and grooves are being cut, or reamed, into the barrel.
Buttoned or cut rifling is generally preferred for it’s accuracy. The forging process can introduce stress into the steel, and this stress is known to reduce accuracy. For most shooters the accuracy loss is negligible, however for bench rest shooters that squeeze every .01% of accuracy out of every component, these CHF process might not be preferred.
D. Barrel Twist Rate
The twist rate of the barrel is the rate of turn the bullet will experience in a given distance. For the purposes of this article only the 223 Remington / 5.56 NATO rounds will be discussed. The information provided will prove a point which can then be applied to any of the nearly countless AR15 chamberings.
First, the twist rate, or ratio, can be defined using “1:7” as an example. What this means is the bullet will experience one complete revolution within a 7” length. Whereas a 1:9 twist rate means the bullet will experience a complete revolution within 9” of length. Which is right? They are both right depending on the needs of the user, and the intended ammunition to be shot from the barrel.
The higher the twist rate, meaning the lower the number of inches taken to complete a revolution, means the more stable the bullet will be leaving the muzzle of the barrel. However it is possible to over stabilize a bullet. For example shooting a 40 or 50 grain varmint round from a 1:7 barrel may cause the bullet to turn at such high revolutions per minute (RPM) that the copper jacket can actually be removed from the lead core. This over stabilization can cause a major loss in accuracy and a loss in ballistic performance for hunting purposes. This is why it’s important to match the barrel to the ammunition, and visa versa.
When it comes down to it, any twist rate will shoot just about any bullet, but when discussing optimization the twist rate should be matched to the ammunition you intend to shoot. The general rule of thumb used is that a 1:7 will shoot bullets in the 70 grain range the best, but will also shoot 60 grain bullets just fine and even most 50 grain bullets without any problems. 1:9 shoots varmint bullets in the 40 and 50 grain range the best, but will also shoot heavier bullets, though at longer distances the heavier bullets will become unstable and cause a loss of accuracy. 1:8 seems to be the modern trade off and is a twist rate gaining popularity. The 1:8 seems to shoot everything well. So the barrel should be matched to the bullet, and if you are not sure; you usually can’t go wrong with a 1:8.
E. Barrel Length will affect many factors on the flight of the bullet including velocity and distance. All things being equal a longer rifle barrel will generally produce more accuracy at longer distances due to the increased velocity, than a shorter pistol barrel for example. It is also important to note that barrels shorter than the carbine length 16” are sometimes considered to be controlled items by the ATF and special permission is required. One must also check short barrel rifles (SBR) with their state, county and city as SBR’s are illegal in some areas.
14.5” barrels are controlled, therefore not owned by most civilians. Though a 14.5” barrel with a permanently attached long muzzle device will usually satisfy laws regard Short Barrel Rifles(SBR). 14.5” barrels are absolutely great for CQB or urban tactical and home defense (with the proper permission). The trade off is a loss of accuracy and terminal performance at longer distances.
16” barrels are the most popular allowing for the most flexibility within the rifle, especially because they do not require any special ATF permission slips. 16” barrels are seemingly good at everything, home defense, three gun competition, most tactical situations, and will even reach out with combat accuracy and effective terminal ballistics to 300 yards or so.
18” barrels are gaining popularity. These barrels are especially popular with three gun shooters. They are usually coupled with mid length or intermediate gas systems giving a great balance of distance, velocity, and reduced recoil impulse while maintaining a manageable weight.
20” barrels were used on the original M16 rifles (the grand father of the modern civilian AR15). The longer barrel will give you more velocity which will increase the effective range of the bullet. Bench rest shooters may desire this increased accuracy thus opt for a longer barrel. However the added weight of the barrel may be a turn off for hunting or tactical purposes and may be clumsy for use in home defense or CQB.
F. AR15 Gas System Length
There are different length gas systems for use on the AR15. The gas system length is sometimes predetermined by barrel length. However in some circumstances different gas system lengths can be used. For less recoil impulse it is preferred to use the longest gas system possible, with the least dwell but still leaving 5” or more of barrel length in front of the gas port (dwell). For comparison purposes the M16 and M4 have dwell of 6.8” and 6.7” respectively.
Pistol length gas system…..
Mid (Intermediate) – slower cyclic rate yield easier follow up shots, shorter dwell time means less pressure on the bolt carrier yielding less wear and tear and less perceived recoil. improved extraction.
G. Gas Port Size is the hole in the barrel which allows gas to bleed off from the barrel, into the gas block, through the gas tube and into the upper receiver and bolt carrier. The size of the gas port is to be specified in conjunction with the gas system length and dwell. The gas port size is one means of tuning the rifle to specific ammunition.
The standard gas port size varies from .059”-.090” and the maximum would be .120” since that is the internal diameter of the standard gas tube. However a larger size gas port diameter can cause the AR15 gas system to become over gassed. The larger size gas ports are not recommended on shorter barrels, and anything over .090 is not usually recommended except with use on an adjustable gas system.
The two trains of thought on the subject; Smaller gas port hole with more dwell OR Larger gas port hole with less dwell.
Common gas port diameters based on barrel length… Please note this chart doesn’t take into account other factors such as barrel diameter at the gas port, bolt carrier weight, buffer weight or buffer spring tension, ammunition or suppressor use. These port sizes will also vary by manufacturer.
- 11.5” barrel….. .810”-.094”
- 14” barrel….. .059”-.086”
- 16” barrel….. .059”-086”
- 20” barrel….. .086”-.096”
- 24” barrel….. .089”
Barrels with a larger diameter at the gas port (.750” diameter) tend to lean towards the middle to larger hole size, while barrels with a small diameter (.625” diameter) at the gas port tend to lean towards the low to middle hole size. Gas port size is one thing that should be considered but shouldn’t be over-thought. Most of the quality manufacturers have done the thinking for you, and unless you specifying a custom made barrel it might not be something necessary to consider too much. When in doubt err on the smaller side of port sizes, as the port size may be enlarged later if necessary.
H. Feed Ramps are available in two basic varieties, standard (aka Rifle) and M4 style feed ramps. The purpose of the feed ramps is to aid the tip of the bullet into the barrel extension, and thus properly chambered. This improved chambering is especially noticeable with ammunition that is pointed at the tip, vs. rounded nose ammunition. It is also important to note that the M4 style feed ramps are best utilized when paired with an upper receiver which also will have m4 feed-ramp extensions. One should never pair a standard/rifle feed-ramped barrel extension with a upper receiver that has m4 feed-ramps.