Twist Rate
Twist rate for rifling is one of the must misrepresented and oversimplified topics in the firearms community. For this reason, we feel compelled to defend our recommendations before we even make them.
What is Twist Rate?
Twist rate describes the bore length that it takes for the rifling to complete one full revolution.
Twist rate is often described as a ratio. For example, a twist rate of 1:7 indicates that the rifling completes a full revolution every 7 inches of bore length.
Twist rate determines the spin rate of the bullet. The faster the twist rate, the faster the bullet will spin as it leaves the bore.
Why Does a Bullet Spin?
A bullet spins because if it doesn’t, it will tumble through the air. This will result in the bullet striking a target however it happens to be oriented when it gets there. It will also result in a dramatic drop in velocity and will reduce the range of the bullet. We spin a bullet to keep it oriented in the right direction. This leverages a phenomenon called Spin Stabilization.
What is Spin Stabilization?
Note that spin stabilization is a complex topic. We will only cover what we need to in support of our conclusions for twist rate. We will publish a dedicated article on spin stabilization soon.
As a projectile moves through the air, it encounters air resistance. This air resistance works against any stabilizing forces that exist in the projectile.
A non-spherical object moving any way other than straight down toward the earth will tend to tumble it encounters air resistance. There are two ways to counter this destabilizing force: Fin Stabilization and Spin Stabilization.
Fin Stabilization
Attaching fins to the back of the projectile will add significant drag to the rear end and shift the Center of Pressure (CP) rearward. Assuming the fins are big enough, the CP will shift behind the Center of Gravity (the gravitationally-neutral locus), and the projectile will be statically stable (meaning that it will orient its axis to the trajectory, all by itself without any additional external force acting upon it). Fin stabilization is seen in archery arrows and rockets. It is not relevant to bullets.
Spin Stabilization
When a projectile is not equipped with fins, the other way to stabilize its flight is to impart a spin on it along its longitudinal axis. By spinning an elongated object along its longitudinal axis, the object experiences Gyroscopic Moment. This is the resistance in changes of axis of rotation due to conservation of angular momentum (this is the “spin stabilization”). The gyroscopic effect will counteract the destabilizing effects of air resistance.
We describe the spin-induced stability of a spinning bullet using a principle called Gyroscopic Stability Factor.
What is Gyroscopic Stability Factor (Sg)?
Gyroscopic Stability Factor (Sg) is a ratio of stabilizing forces to destabilizing forces for a given bullet, at a given velocity, spinning at a given rate as it travels through the air.
The primary stabilizing force of a spinning bullet is it’s Gyroscopic Moment (the resistance in changes of axis of rotation due to conservation of angular momentum). This is affected by the spin rate and the distribution of mass in the bullet (bullet length and diameter).
The primary destabilizing force is air resistance. This air resistance is caused by air density, which is a combined effect of altitude, ambient temperature, and relative humidity. As the bullet travels through the air, it pushes its way through the air, and the air pushes back. This “ballistic wind” does not apply pressure evenly to the surface of the bullet. Depending on the shape and length of the bullet, the relationship between the attitude and the trajectory of the bullet, and the velocity of the bullet, this aerodynamic force can have more or less impact on the stability of the bullet. We will not get into excruciating detail here, but suffice to say that this force is basically trying to get the bullet to do a back flip and tumble through the air.
The ratio of stabilizing forces and destabilizing forces determines whether a bullet is stable or not.
How Do I Calculate Gyroscopic Stability Factor (Sg)?
The easy answer is to use a calculator like the one available from Berger Bullets (found HERE). This calculator uses the Miller Twist Rule with corrections for muzzle velocity, altitude, and ambient temperature.
The classical formula for calculating gyroscopic stability factor is Greenhill’s Rifling Formula.
Donald Miller simplified Greenhill’s formula to be more accessible and easier to calculate. This is known as the Miller Twist Rule. The formulae for both can be seen below.
What is a "Good" Gyroscopic Stability Factor (Sg)?
A gyroscopic stability factor of 1.000 indicates that the destabilizing forces and the stabilizing forces are equal. If the Sg is less than 1.000, the destabilizing forces are greater than the stabilizing forces, so the bullet is not gyroscopically stable and will not perform well under any circumstances. If the Sg is greater than 1.000, the destabilizing forces are less than the stabilizing forces, so the bullet is technically gyroscopically stable.
An Sg between 1.000 and 1.500 indicates marginal gyroscopic stability at the muzzle; it is not ideal, but the bullet might fly ok. You want an Sg of 1.500 or higher; this will guarantee gyroscopic stability under most conditions and you will achieve the maximum ballistic coefficient. The generally accepted “safe” Sg is 2.0, so that is what you should be targeting.
Some experts claim that excessively high Sg can lead to poor accuracy at long distances. The idea is that the axis of a bullet that is spinning too fast will not adapt along the trajectory; it will stay oriented to the bore axis along its entire trajectory and the angle of attack will progressively grow. While this may be true, this concern is born from application of over-stabilization to high angle artillery, where an extreme bore attitude and an over-stabilized shell will result in a shell hitting butt-first (which is a problem when you need it to hit nose-first to go boom). We don’t discount the fact that the same phenomenon occurs with small arms, but its not as serious as some make it out to be. If you are in the precision long range game, keep the Sg close to 2.0 and below 3.0; you should be fine.
What is the Ideal Twist Rate for My Barrel?
This is where the oversimplification and misinformation happens.
The ideal twist rate is dependent on the bullet and cartridge.
Many sites and forum contributors out there make blanket statements like, “for 62 grain bullets you want 1 in 7 inches” or “1 in 7 inches is good 60-80 grain bullets”; you cannot separate the bullet from the cartridge for this discussion. The ideal twist rate is based on the Gyroscopic Stability Factor. As we have seen above, the Sg for bullets of the same weight can be different, depending on the the bullet length and diameter, based on the environmental conditions, and based on the muzzle velocity of the cartridge out of a given barrel length.
If you are willing to take our word for it, jump to the twist rate tables, below. They have been populated based on values from either the Applied Ballistics, Strelok Pro, or JBM bullet databases (Tables 1; as indicated) and calculation of gyroscopic stability factor using the Berger Twist Rate Stability Calculator for commercially available factory ammunition (Tables 2).
If you’re not willing to take our word for it (or if you want to double check our calculations), we encourage you to go do the calculation yourself; if you are using the same numbers and the same calculator, you’ll get the same result.
Tables 1 in the tabs below lists the ballistic information for each cartridge used to calculate Sg.
Table 2 lists the the Sg for each of the cartridges at various twist rates.
9mm Luger
Short Answer: Don’t worry about it. 9mm AR barrels come standard in 1:10″ twist and that is way more than enough to stabilize any 9mm bullet.
Any barrel with a twist rate faster than 1:29″ will stabilize a Federal 115gr FMJ 9mm bullet. The Sg for this round fired out of a short barrel with a standard 1:10″ twist is around 12.3, which is extremely high, and it will only climb with a longer barrel (due to increased velocity).
Any barrel with a twist rate faster than 1:32″ will stabilize a Federal 147gr FMJ 9mm bullet. The Sg for this round fired out of a short barrel with a standard 1:10″ twist is around 15.0, which is extremely high, and it will only climb with a longer barrel (due to increased velocity).
Suffice to say, your 9mm bullet will be in the dirt long before it loses gyroscopic stability in flight.
.223 Remington/5.56 NATO
Short Answer: If you don’t want to glance down at the tables below to find the best twist rate for your cartridge, go with 1:7” twist. As long as you’re not shooting frangible bullets, you’ll be fine.
Table 1: Bullet/Cartridge Data
| Cartridge Number | Bullet | Bullet Weight | Bullet Length* | G1 B.C.* | G7 B.C.* | M.V.** |
|---|---|---|---|---|---|---|
| 1 | Hornady FMJ 5.56 (M193) | 55 | 0.741 | 0.242 | 0.124 | 3240 (20") |
| 2 | Federal XM193 | 55 | 0.745 | 0.243 | --- | 3300 |
| 3 | Federal AE FMJ 5.56 (XM193) | 55 | 0.745 | 0.246 | --- | 3165 |
| 4 | Federal XM855 | 62 | 0.907 | 0.304 | --- | 3000 |
| 5 | Black Hills Sierra MK | 69 | 0.9 | 0.305 | --- | 2875 |
| 6 | Black Hills Sierra TMK | 69 | 0.982 | 0.365 | --- | 2875 |
| 7 | Berger Target BT | 73 | 0.961 | 0.350 | 0.179 | 2820 (20") |
| 8 | Hornady ELD Match | 73 | 1.049 | 0.410 | 0.210 | 2790 (24") |
| 9 | Hornady SP Match | 75 | 0.981 | 0.395 | --- | 2910 |
| 10 | Black Hills Sierra MK | 77 | 0.994 | --- | 0.19 | 2750 |
| 11 | Berger Tactical OTM | 77 | 1.011 | 0.375 | 0.192 | 2750 (20") |
Table 2: Gyroscopic Stability for Select Cartridges
| Cartridge (Table 1) | Bullet Weight | 1:14" | 1:12" | 1:10" | 1:9" | 1:8" | 1:7" |
|---|---|---|---|---|---|---|---|
| 1 | 55 | 0.999 | 1.36 | 1.96 | 2.42 | 3.06 | 3.99 |
| 2 | 55 | 0.990 | 1.35 | 1.94 | 2.39 | 3.03 | 3.96 |
| 3 | 55 | 0.976 | 1.33 | 1.91 | 2.36 | 2.99 | 3.90 |
| 4 | 62 | 0.615 | 0.838 | 1.21 | 1.49 | 1.88 | 2.46 |
| 5 | 69 | 0.691 | 0.940 | 1.35 | 1.67 | 2.11 | 2.76 |
| 6 | 69 | 0.537 | 0.730 | 1.05 | 1.30 | 1.64 | 2.15 |
| 7 | 73 | 0.601 | 0.817 | 1.18 | 1.45 | 1.84 | 2.40 |
| 8 | 73 | 0.464 | 0.631 | 0.909 | 1.12 | 1.42 | 1.86 |
| 9 | 75 | 0.587 | 0.799 | 1.15 | 1.42 | 1.80 | 2.35 |
| 10 | 77 | 0.570 | 0.775 | 1.12 | 1.38 | 1.74 | 2.28 |
| 11 | 77 | 0.542 | 0.738 | 1.06 | 1.31 | 1.66 | 2.17 |
Table 2 Assumptions:
- Muzzle Velocity: Equal to cartridge manufacturer’s claim.
- Ambient Temperature: 59°F
- Altitude: 0 ft (sea level)
.300 Blackout
Short Answer: If you don’t want to glance down at the tables below to find the best twist rate for your cartridge, go with 1:7” twist for subsonic ammo or 1:8″ twist for all other ammo. These happen to be the twist rates that you will find in most factory barrels.
Table 1: Bullet/Cartridge Data
| Cartridge Number | Bullet | Bullet Weight | Bullet Length* | G1 B.C.* | G7 B.C.* | M.V.** |
|---|---|---|---|---|---|---|
| 1 | Winchester USA FMJ | 125 | 1.1 | 0.272 | --- | 2185 |
| 2 | Hornady AG HP | 125 | 1.1 | 0.320 | --- | 2175 |
| 3 | Federal AE FMJBT | 150 | 1.260 | 0.406 | --- | 1900 |
| 4 | Hornady SUB-X | 190 | 1.6 | 0.437 | --- | 1050 |
| 5 | Hornady A-Max | 208 | 1.532 | 0.635 | 0.325 | 1020 |
| 6 | Federal Suppressor OTM | 220 | 1.6 | 0.650 | --- | 1000 |
* Based on the values from the manufacturer, Applied Ballistics bullet database, Strelok Pro bullet database, or JBM bullet database.
** Based on manufacturer’s specifications. If provided, the length of barrel used to collect the velocity is indicated in parentheses.
Table 2: Gyroscopic Stability for Select Cartridges
| Cartridge (Table 1) | Bullet Weight | 1:11" | 1:10" | 1:9" | 1:8" | 1:7" | 1:6" |
|---|---|---|---|---|---|---|---|
| 1 | 125 | 1.89 | 2.28 | 2.82 | 3.57 | 4.66 | 6.34 |
| 2 | 125 | 1.88 | 2.28 | 2.81 | 3.56 | 4.65 | 6.33 |
| 3 | 150 | 1.46 | 1.77 | 2.18 | 2.76 | 3.61 | 4.92 |
| 4 | 190 | 0.759 | 0.918 | 1.13 | 1.43 | 1.87 | 2.55 |
| 5 | 208 | 0.934 | 1.13 | 1.40 | 1.77 | 2.31 | 3.14 |
| 6 | 220 | 0.864 | 1.05 | 1.29 | 1.63 | 2.13 | 2.90 |
Table 2 Assumptions:
- Muzzle Velocity: Equal to cartridge manufacturer’s claim.
- Ambient Temperature: 59°F
- Altitude: 0 ft (sea level)
.308 Winchester
Short Answer: If you don’t want to glance down at the tables below to find the best twist rate for your cartridge, go with 1:10” twist. This is pretty much the standard twist rate for .308 Winchester, and you can see why by looking at Table 2. Some very heavy bullets (230 gr and 250 gr) may like faster twists, but you probably won’t be able to feed them through an AR due to the excessive cartridge length.
Table 1: Bullet/Cartridge Data
| Cartridge Number | Bullet | Bullet Weight | Bullet Length* | G1 B.C.* | G7 B.C.* | M.V.** |
|---|---|---|---|---|---|---|
| 1 | PMC FMJBT | 147 | 1.16 | 0.398 | --- | 2780 |
| 2 | Hornady SST | 150 | 1.153 | 0.365 | 0.187 | 3000 |
| 3 | Hornady SST | 165 | 1.274 | 0.438 | 0.224 | 2840 |
| 4 | Sierra MatchKing | 168 | 1.215 | 0.417 | 0.214 | 2650 |
| 5 | Barnes TTSX | 168 | 1.416 | 0.455 | 0.233 | 2700 |
| 6 | Hornady ELD Match | 168 | 1.279 | 0.511 | 0.261 | 2700 |
| 7 | Hornady SP ELD Match | 168 | 1.279 | 0.511 | 0.261 | 2840 |
| 8 | LC M118LR OTM | 175 | 1.240 | 0.474 | 0.243 | 2580 |
| 9 | Sierra MatchKing | 175 | 1.240 | 0.479 | 0.241 | 2600 |
| 10 | Berger Tactical OTM | 175 | 1.258 | 0.514 | 0.263 | 2668 |
| 11 | Barnes Match Burner | 175 | 1.335 | 0.517 | 0.265 | 2600 |
| 12 | Hornady ELD Match | 178 | 1.322 | 0.540 | 0.276 | 2600 |
| 13 | Berger Tactical Juggernaut OTM | 185 | 1.346 | 0.555 | 0.284 | 2608 |
* Based on the values from the manufacturer, Applied Ballistics bullet database, Strelok Pro bullet database, or JBM bullet database.
** Based on manufacturer’s specifications. If provided, the length of barrel used to collect the velocity is indicated in parentheses.
Table 2: Gyroscopic Stability for Select Cartridges
| Cartridge (Table 1) | Bullet Weight | 1:12" | 1:11" | 1:10" | 1:9" | 1:8" | 1:7" | 1:6" |
|---|---|---|---|---|---|---|---|---|
| 1 | 147 | 1.73 | 2.06 | 2.50 | 3.08 | 3.90 | 5.10 | 6.94 |
| 2 | 150 | 1.85 | 2.20 | 2.66 | 3.28 | 4.16 | 5.43 | 7.39 |
| 3 | 165 | 1.50 | 1.78 | 2.16 | 2.66 | 3.37 | 4.40 | 5.99 |
| 4 | 168 | 1.71 | 2.03 | 2.46 | 3.04 | 3.84 | 5.02 | 6.83 |
| 5 | 168 | 1.10 | 1.31 | 1.59 | 1.96 | 2.48 | 3.24 | 4.41 |
| 6 | 168 | 1.48 | 1.76 | 2.13 | 2.63 | 3.33 | 4.35 | 5.93 |
| 7 | 168 | 1.51 | 1.79 | 2.17 | 2.68 | 3.39 | 4.43 | 6.03 |
| 8 | 175 | 1.66 | 1.98 | 2.39 | 2.96 | 3.74 | 4.89 | 6.65 |
| 9 | 175 | 1.67 | 1.98 | 2.40 | 2.96 | 3.75 | 4.90 | 6.67 |
| 10 | 175 | 1.61 | 1.92 | 2.32 | 2.87 | 3.63 | 4.74 | 6.45 |
| 11 | 175 | 1.35 | 1.60 | 1.94 | 2.39 | 3.03 | 3.96 | 5.39 |
| 12 | 178 | 1.41 | 1.68 | 2.03 | 2.50 | 3.17 | 4.14 | 5.64 |
| 13 | 185 | 1.39 | 1.66 | 2.00 | 2.47 | 3.13 | 4.09 | 5.57 |
Table 2 Assumptions:
- Muzzle Velocity: Equal to cartridge manufacturer’s claim.
- Ambient Temperature: 59°F
- Altitude: 0 ft (sea level)
6.5 Creedmoor
Short Answer: If you don’t want to glance down at the tables below to find the best twist rate for your cartridge, go with 1:8” twist. This is pretty much all you will find in factory barrels. If you are having a barrel custom-made, you are probably best off with a 1:7″ twist.
Table 1: Bullet/Cartridge Data
| Cartridge Number | Bullet | Bullet Weight | Bullet Length* | G1 B.C.* | G7 B.C.* | M.V.** |
|---|---|---|---|---|---|---|
| 1 | Federal OTM | 120 | 1.17 | 0.406 | --- | 2900 |
| 2 | Hornady ELD Match | 120 | 1.203 | 0.466 | 0.238 | 2910 |
| 3 | Berger Hybrid Hunter VLD (Federal) | 130 | 1.321 | 0.564 | 0.289 | 2875 |
| 4 | Sierra MatchKing HPBT (Federal) | 140 | 1.303 | 0.529 | 0.271 | 2675 |
| 5 | Hornady ELD Match | 140 | 1.380 | 0.589 | 0.296 | 2710 |
| 6 | Berger Hybrid Target (Federal) | 140 | 1.402 | 0.608 | 0.306 | 2925 |
| 7 | Hornady ELD Match | 147 | 1.433 | 0.629 | 0.316 | 2695 |
* Based on the values from the manufacturer, Applied Ballistics bullet database, Strelok Pro bullet database, or JBM bullet database.
** Based on manufacturer’s specifications. If provided, the length of barrel used to collect the velocity is indicated in parentheses.
Table 2: Gyroscopic Stability for Select Cartridges
| Cartridge (Table 1) | Bullet Weight | 1:10" | 1:9" | 1:8" | 1:7" |
|---|---|---|---|---|---|
| 1 | 120 | 1.51 | 1.86 | 2.36 | 3.08 |
| 2 | 120 | 1.39 | 1.72 | 2.18 | 2.84 |
| 3 | 130 | 1.14 | 1.41 | 1.79 | 2.33 |
| 4 | 140 | 1.25 | 1.55 | 1.96 | 2.55 |
| 5 | 140 | 1.06 | 1.31 | 1.66 | 2.17 |
| 6 | 140 | 1.04 | 1.29 | 1.63 | 2.12 |
| 7 | 147 | 0.997 | 1.23 | 1.56 | 2.04 |
Table 2 Assumptions:
- Muzzle Velocity: Equal to cartridge manufacturer’s claim.
- Ambient Temperature: 59°F
- Altitude: 0 ft (sea level)
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