When the U.S. military first adopted the M16 rifle in the 1960s, the M193 cartridge and its 55-grain bullet was standard. The earliest issued variations of Eugene Stoner's "Black Rifle" came with relatively slow rifling twist rates of 1-in-14 inches. Shortly thereafter, nearly all M16s and M16A1s were being issued with faster 1-in-12-inch twist barrels.
In more modern times, bullets for military and civilian use have migrated to longer, heavier designs and twist rates have been altered to stay in-sync with this progression. With so many bullets and twist rates available these days, keeping track of which ammunition is compatible with your barrel can be overwhelming.
Before we get into the weeds on individual twist rates, let's take a minute to discuss rifling in general. "Rifling" are the lands and grooves impregnated into the barrel's interior that impart spin on a projectile as it travels down the bore. This spin stabilizes the bullet in flight, much the way a football is "spiraled" by a quarterback.
Determining proper twist is a factor of bore diameter, velocity, bullet weight and even bullet construction. There is no "golden" twist rate for all firearms. Civil war muskets such as the 1861 Springfield used extremely slow twist rates (1-in-78") to fire heavy lead bullets with relatively good accuracy, while modern AR-15-style rifles use barrels as fast as 1-in-7 to stabilize long-for-caliber projectiles.
Conventional wisdom taught us that slower twist rates wouldn't properly-stabilize a bullet, causing it to yaw. On the other hand, faster rates could over-stabilize lighter bullets, causing similar problems. This is correct in theory — however, modern ballisticians have pretty much debunked the over-stabilization theory as a practical matter. All things being equal, it is better to have too much twist than not enough.
Don't misunderstand me; serious disparities in bullet weight to twist rate can cause poor accuracy, decrease velocity and potentially compromise a weak bullet's structural integrity. I'd just prefer to air on the side of faster-twist barrels—especially with carbine-length barrels.
Over the past decade or so, military rifles chambered in 5.56mm have migrated in two directions:
- Shorter guns such as 10.5-inch carbines emerged as the fight put troops in and out of vehicles and into close-quarter fighting.
- Accurized, Special Purpose Rifles (SPRs) were issued to fill the long-range overwatch niche between standard carbines and specialized sniper rifles.
In both cases, heavy bullets evolved as solutions for terminal and external ballistic issues. Shorter guns needed the added bullet mass to compensate for lost velocity, while the SPRs used heavier, high ballistic-coefficient bullets to gain long-range performance.
The two bullets that rose to the top of the heap were the Sierra 77-grain BTHP/OTM and the Barnes 70-grain TSX "Brown Tip." Both bullets are excellent at their intended tasks and neither will perform appropriately in the slower-twist barrels that were standard for decades.
Let's take a look at the commonly-available twist rates used in-conjunction with the 5.56mm NATO/.223 Remington chambering.
1-in-14 Inches Twist Rate
You're unlikely to find a 1-in-14 barrel on any AR-15 produced in recent years, but they do exist and you may encounter them on a bolt action rifle on occasion. This is the least-versatile twist rate you'll see in the 5.56x45mm/.223 Remington chambering. Although 1-in-14 barrels can stabilize 55-grain bullets used by the original M16, it's really better suited for bullets up to 50- and 52-grains—most of which fall into the varmint and target category. If achieving maximum velocity with light bullets is your goal, this may fit for your needs — Winchester's 45-grain JHP load is a screamer at 3,600 feet per second.
1-in-12 Inches Twist Rate
This is the slowest twist still seen in large numbers on AR-15s and other .223s. Though far better than the 1-in-14 twist, it is still unsuited for some of the premium loads developed over the past decade. Conventional wisdom suggests this twist rate is perfect for bullets in the 55-to 60-grain range, though most will stabilize the common 62-grain FMJ rounds. If you hunt prairie dogs or coyotes with lightweight .223 bullets, this twist rate will do fine for your needs. Doubletap's 55-grain Nosler Ballistic Tip load generates 3,300 feet per second of velocity out of a 22-inch barrel, and will easily stabilize in this twist rate.
1-in-10 Inches Twist Rate
My first centerfire rifle was a Ruger Mini-14 with a 1-in-10 twist. This is a good twist rate for lighter bullets and will also generally stabilize projectiles up to 69 grains, such as Federal Premium's Sierra MatchKing BTHP load. If you're happy with 55- and 62-grain FMJ bullets, you don't need any more twist than this. In my mind, however, the 1-in-10 twist is just a bit too restrictive.
1-in-9 Inches Twist Rate
This is the beginning of the road for the shooter wanting to take advantage of the heavy bullet trend. The 1-in-9 is a great compromise twist rate—not too fast to cause problems with the 55-grain Bullets, but fast enough to stabilize all but the heaviest bullets under most circumstances. This twist will stabilize most traditional bullets up to 75-grains, and monolithics up to 70-grains — but they do so right at the edge of the envelope so not all rifles will do it. My personal 16-inch Rock River Arms carbine with a 1-in-9 twist does fine with ASYM's Tactical Match Grade 77-grain OTM load, but has shown signs of instability with handloads using the 70-grain Barnes TSX — unless the bullet is pushed to maximum velocity. With longer barrels and the commensurate faster velocities, this twist can be more forgiving.
1-in-8 Inches Twist Rate
For a 16-inch general-use carbine, the 1-in-8 twist is about as versatile as it gets. This twist rate will comfortably stabilize bullets up to 80-grains, and the excellent 75- and 77-grain bullets also work great at a wider spectrum of velocities—which means barrel length isn't critical. My 3-gun rifle, built by my friend Iain Harrison, wears an 18-inch, 1-in-8 twist White Oak Armament barrel and shoots just about anything well.
1-in-7 Inches Twist Rate
This is the twist chosen by the military since the switch was made to the M16A2 — and the 62-grain M855 cartridge — in the 1980s. This twist is found on the M4 carbine, the M16A4, the Mk12 Special Purpose Rifle and even the HK416. Its ability to stabilize tracer rounds in-flight is one of the reasons that the military chose this twist rate.
This barrel will stabilize bullets of up to 90 grains, and can handle the 70- to 77-grain bullets at just about any velocity, which makes it well suited for carbines with very short barrels. If you want a Mil-Spec clone, the 1-in-7 twist is the way to go.
I currently own two AR-15s with 1-in-7 twist rates — a 10.3-inch Mk18 Mod. 1 and an 18-inch Mk12 Mod. 1 — both made by Monty LeClair at Centurion Arms. To illustrate how velocity and twist rates can be the ying and the yang of bullet stability and accuracy, I fired a variety of loads side-by-side from the two rifles. The 10.3-inch SBR shoots anything from 55-grain FMJ to 77-grain OTM with excellent accuracy, while the 18-inch Mk12 is scary accurate with the 70-, 75- and 77-grain bullets—but won't shoot 55-grain or 62-grain FMJs worth a damn.
Matching the rifling twist in your rifle or carbine to the appropriate ammunition won't guarantee great accuracy, but it will ensure the bullet is properly stabilized in flight. On the other hand, using a bullet that's too heavy for your barrel's twist is a virtual promise of poor accuracy and ineffective terminal performance. If you're struggling with the accuracy of your modern sporting rifle, be sure you've properly matched your ammunition to the barrel's twist.