Barrel length and twist rate can make or break your custom AR build. A barrel that's too short won't provide good velocity and accuracy will suffer. A bad twist rate's even worse: With the right barrel it won't matter, your rounds will over-stabilize or fail to stabilize at all, resulting in bullet tumble and wonky physics.
Barrel Length vs. Velocity
A lot of gun snobs on forums like to say barrel length doesn't directly affect accuracy. It does. Velocity, which is perhaps the most important factor contributing to accuracy, is generated by the cartridge accelerating down the barrel. The longer the barrel, the more time the cartridge's gunpowder has to propel the bullet up to higher velocity. Velocity affects maximum distance and bullet drop, which are two of the greatest factors in shooting accurately.
Finding the Best Barrel Length
Unless you're interested in building the longest rifle possible to sit at a bench and chase your cartridge's max distance, finding the "best" barrel length winds up becoming a compromise between length and velocity. This holds especially for true for AR-type rifles and pistols, which (regardless of cartridge) are intended to configured as lightweight, maneuverable firearms that balance accuracy with weight and size.
The standard for determining the best barrel length, then, requires looking at the cartridge you'll be chambering. You want a barrel that'll get your cartridge close to maximum velocity with the shortest barrel possible. Determining that means looking at velocity and barrel length together. That's what we'll be diving into below.
Twist Rate vs. Accuracy
Next to barrel length, twist rate is just as important for accuracy. We're talking about the rotation of the rifling inside the barrel. Twist rate is expressed as a ratio of two numbers to indicate how often the rifling completes a full rotation. Rates expressed as "1:7" or "1:10" indicate the rifling completes a rotation every seven or ten inches.
Unless you plan on shooting a single bullet weight within your chosen cartridge's available variants, you will have to compromise on twist rate like you will with barrel length. A twist rate that's optimal for a lighter bullet may fail to stabilize a heavier load. And a heavier bullet's twist rate could "over-spin" a lighter bullet, decreasing accuracy.
So, how do we determine the best twist rate?
Gyroscopic stability is to twist rates what velocity is to barrel length: An indicator of what is best for a chosen bullet weight. Abbreviated GSF, these factors show whether a twist rate is too fast or too slow for a particular bullet. We won't get into the complicated calculations. Instead, use the guidelines below when reviewing gyroscopic factors and bullet weights:
- GSFs below 1 are unstable, or too slow.
- GSFs above between 1.1 and 1.3 is marginal.
- GSFs of 1.4 to 2.0 are stable and optimal.
- GSFs of 1.5+ are best for long-range shooting.
- GSFs of 2.1 to 3.5 may cause some spin drift at very long distances.
- GSFs of 3.6 and above may cause the bullet to drift at shorter distances.
- GSFs greater than 6.0 may rarely cause the bullet to yaw or tumble in flight.
Ballistic coefficient - the ability of a bullet to overcome air resistance in flight - is directly affected by gyroscopic factors. For every GSF point below 1.5, you will lose 3% ballistic coefficient.
Always pick the faster twist.
When it comes time to pick a twist rate for your cartridge, always pick a rate that may be a bit too fast for lighter bullets over one that may be too slow for heavier bullets. The heaviest (and thus most accurate) bullets benefit the most from faster twist rates, and a fast rate will affect accuracy less on a lighter load, compared to running a twist rate that's too slow for a heavier load.
AR-15 Barrel Lengths and Twist Rates
While the AR-15 can chamber quite a few cartridges, we're focusing on those which are the most popular: 5.56 NATO, .223 Remington, 300 Blackout, and 6.5 Grendel. We'll cover the cartridges that fit in the .308 and 9mm receivers below, too.
5.56 NATO / .223 Remington
5.56 and .223 are virtually identical, as far as barrel and twist requirements go. Consider them the same, here.
Best Barrel Length
If you're wondering why all AR-15 rifle kits come with a 16" barrel, it's not just because this is the minimum legal barrel length for any rifle. It's also the optimal length. It affords the most velocity with the shortest barrel. Adding any more length beyond 16" will only provide a few dozen extra feet per second of velocity, on average, and accuracy won't benefit. This was proven decades ago, when the U.S. Military modified the M16 and reduced its barrel length from 20", eventually settling on a 14.5" barrel with the M4 Carbine.
Best Twist Rate
This first gyroscopic factor chart shows just how much the bullet weight and variant of the cartridges matters when it comes to picking a twist rate for your AR. The two most common variants you'll wind up shooting are the 55- and 62-grain M193 and M855. These are found in most big-box sporting good stores. They're also the most affordable.
If these two cartridges are the ones you plan on shooting, we recommend a 1:8 twist rate. The 1:8 rate provides good stability for the 62-grain load, which can improve accuracy at great distances. And, if you ever want to chamber some Sierra MatchKings, the 1:8 rate can get the job done without over-stabilizing those lighter 55- and 62-grain loads.
Why so many AR-15s still use a 1:7 twist rate
The 1:7 rate is mil-spec, and it's the best rate for the most accurate 80- and 90-grain loads. It's what the U.S. Military uses in the M4 and M16. The commercial market and AR-15 owners assume that whatever the military uses must be the best choice. Our chart above shows that this is mostly true, and it's why most of our build kits come with this rate.
If you opt for the 1:7, you can still shoot 55- and 62-grain loads with accuracy. The rate is not high enough to over-stabilize these rounds, and you won't notice any loss in accuracy downrange, even if the chart above says this rate is "too fast."
300 Blackout, Supersonic & Subsonic
The .300 AAC Blackout is the AR-15 cartridge most affected by barrel length and twist rate. That's because it's really two cartridges, with two separate purposes, in one: Supersonic loads tend to be twice as light and fast as subs and are made for long-distance shooting. Subs are made for close-range shooting, usually with a suppressor.
Best Barrel Length
Our 300 Blackout Build Guide goes into more detail, but the summary is that both supersonic and subsonic loads do well with a 9" barrel. This is the shortest length you can install while achieving near-maximum velocity. Some shooters choose to go with an 11.5" barrel, but any length beyond this will not yield more velocity or accuracy for either load.
Best Twist Rate
The 300 BLK's gyroscopic factors paint the story well: Twist rates that stabilize supersonic loads (112- to 125-grain) will tend to be a bit slow for stabilizing heavy subsonic loads, like 208- to 230-grainers. The inverse is also true: A twist rate that's optimal for subsonics, like the 1:8 rate, will be fast for supersonics.
Speaking of the 1:8 rate, this will be best for shooters who want to shoot mostly subsonics while occasionally chambering supersonic loads. This rate isn't too fast for supers, so it won't over-stabilize your rounds, and the maximum effective range of supersonics isn't great enough for the fast spin to cause any loss of accuracy. If, for some reason, you plan on building a supersonic-only 300 Blackout AR, the 1:12 rate is best.
This build kit comes with a 9" barrel and 1:8 twist rate for a subsonic build.
The 6.5 Grendel AR was made for long-distance shooting (500 to 800 yards) and requires quite a long barrel to achieve high accuracy. Unlike other AR-15 cartridges, you want to pursue maximum velocity - and therefore, maximum barrel length - in order to shoot accurately at the greatest maximum distance.
Best Barrel Length
Since barrel length is more important to Grendel than other AR-15 cartridges, we're looking at a velocity chart first. The chart above compares velocities from the muzzle to 1,000 yards using a 129-grain load and barrels that measure 14.5", 16", 20" and 24". Between all tested barrels, these are the reported velocities at 800 yards. This is considered Grendel's max effective range based on lethality (retained energy) and ballistic coefficient (chance to hit):
- 14.5" Barrel: 1,180 FPS
- 16" Barrel: 1,216 FPS
- 20" Barrel: 1,259 FPS
- 24: Barrel: 1,304 FPS
The 129-grain Hornady SST reports an optimal muzzle velocity of 2,238. Between all barrels, these are the reported muzzle velocities:
- 14.5" Barrel: 2,238 FPS
- 16" Barrel: 2,310 FPS
- 20" Barrel: 2,391 FPS
- 24: Barrel: 2,470 FPS
The cartridge's optimal muzzle velocity is achieved with the shortest tested barrel, 14.5". At this length, velocity at 800 yards is 10% less than it is with a 24" barrel, or around 124 FPS slower. While these data don't constitute an apparently large difference in ballistics, the corresponding bullet drop is what matters most. These are the reported bullet drops for all barrels at 800 yards:
- 14.5" Barrel: -121"
- 16" Barrel: -113"
- 20" Barrel: -105"
- 24: Barrel: -97"
The 24" barrel affords two feet less drop at this distance, which is significant. For the typical 1/4-click MOA turrets on most rifle scopes, that's 12 fewer clicks of adjustment. The differences get even greater at 1,000 yards: a 24" barrel yields 5.5 feet (66") less bullet drop compared to a 14.5" barrel. That's 26 fewer clicks on the same turrets.
So, what's the best barrel length for Grendel?
We won't say that a 24" barrel is the best barrel length. This adds significant weight and length to your rifle, whereas a 20" barrel only drops 8" more at 800 yards, and only 21" more at 1,000 yards. Overall, a 20" barrel is the best length for achieving optimal velocity and accuracy with 6.5 Grendel, especially if you plan on shooting within its advertised 800 yards of maximum effective distance.
Optimal Twist Rate
Since the Grendel cartridge retains significantly similar velocities across its small spread of grain weights (which average 90 to 130 grains), one twist rate is effective for generally all loads. This would be 1:8. This rate provides the best gyroscopic factors for the heaviest and most accurate loads without over-stabilizing lighter 90-grain loads.
AR-10 Twist Rates and Barrel Length
The AR-10 and the LR-308 rifle (which is commonly misidentified as the AR-10) chamber 7.62x51 NATO, .308 Winchester and, more recently, 6.5 Creedmoor. We'll cover all three.
7.62x51 NATO / .308 Winchester
Consider .308 Winchester and 7.62 NATO identical for the sake of barrel length and twist rate. They use the same bullet weights and powder.
Best Barrel Length
The 7.62 NATO and .308 Win cartridges require a 20" barrel for optimal velocity and accuracy. No longer barrel will afford any additional velocity. Increasing length to a 24" barrel provides just 80 extra feet per second, which would contribute less than 3% more speed to these cartridges' average velocities.
The Best Twist Rate
The gyroscopic factors for these .30-cal loads speaks for itself. There is, in this case, just one optimal twist rate for all loads, and that's 1:12. This rate provides the best factors for everything from the most common 147-grain loads, to the heavier 180-grain or higher Sierra MatchKings you might chamber for maximum accuracy.
The 6.5 Creedmoor AR is to the .308 and 7.62 NATO what 6.5 Grendel is to 5.56/.223: A more accurate long-rate cartridge that uses the same rifle platform, shooting a 0.264"-diameter bullet with a lot of powder packed behind it. 6.5 Creedmoor requires perhaps the longest barrel of any AR-type rifle cartridge, so finding out the shortest possible length is critical if you don't want a rifle that's as long as you are tall.
Optimal Barrel Length
Thankfully, some interesting data exist that show the Creedmoor cartridge actually begins to slow down if you go with a barrel that's too long. That magical barrel length happens to be 24". Illustrated above, it's been shown a 142-grain Sierra MatchKing will fall from 2,683 FPS at 24" to 2,679 FPS if you use a 25" barrel, and velocity continues to decline as you increase barrel length even more.
Optimal Twist Rate
It should come as no surprise that, like Grendel, the Creedmoor cartridge requires the same 1:8 twist rate for that 0.264" bullet downrange. Simple enough.
AR9 (9mm) Barrel Lengths and Twist Rates
Although the 9mm cartridge has long been chambered in handguns and shorter-barreled firearms, it's become arguably the most popular round among AR owners who want to build an effective pistol-caliber AR called the AR9. Like 300 BLK, the 9mm cartridge can serve multiple roles. It can function as a long-distance supersonic load, or a shortrange subsonic load that works well with suppressors. Barrel length plays a critical role in in the AR9, because it largely determines whether many 9mm loads will remain subsonic, or head downrange at supersonic velocities.
Note: For a cartridge to remain subsonic, it must exit the muzzle below 1,125 FPS.
Best Barrel Length
Our 9mm velocity chart identifies the optimal velocities with the shortest barre length, which are highlighted in green. The velocities that illustrate the maximum barrel length you can use before velocities decrease are highlighted in yellow. Averaging the chart, we can see that a 9" barrel length is generally optimal, as many cartridges begin to experience lesser velocities beyond this length.
Want to go subsonic?
To reliably keep most cartridges at subsonic velocities for suppressed fire, you'll need to dramatically reduce barrel length from 9". Referencing the chart again, we find that you'd need a 5" barrel to keep the most popular 147-grain loads subsonic, and a 4" barrel to get the lightest (115-grain) loads subsonic.
Best Twist Rate
For its wide range of velocities and grain weights, 9mm still gets to enjoy just a single, optimal twist rate, no chart required. That would be 1:10. Whether you're running a short subsonic or long supersonic configuration, this rate provides the best gyroscopic factors for virtually all available 9mm loads.
Here's a short bulleted list that covers the best barrel lengths and twist rates for each caliber we covered:
- 5.56/.223: 16" barrel, 1:8 twist rate.
- 300 BLK: 9" to 11.5" barrel, 1:8 or 1:7 twist rate.
- 6.5 Grendel: 20" barrel, 1:8 twist rate.
- 7.62/.308: 20" barrel, 1:12 twist rate.
- 6.5 Creedmoor: 24" barrel, 1:8 twist rate.
- 9mm Para: 9" barrel, 1:10 twist rate.