AR-15 Twist Rate
Recommended twist, the factory rates you will find, and a calculated stability chart for the bullets AR-15 actually shoots.
AR-15 barrels ship in four common twist rates, and the marking on the barrel tells you which bullets the rifle was built around. This chart runs the Miller stability rule across the bullet weights you can actually buy.
For a general-purpose rifle choose 1:8". For a heavy-bullet precision upper choose 1:7". Reserve 1:12" for dedicated varmint work with 40 to 55 grain bullets.
For reference, a 62 grain bullet at 2950 fps in a 1:8" barrel comes out at a stability factor of 1.85.
AR-15 twist rate chart
Required twist is the rate that puts each bullet at a stability factor of 1.5. The remaining columns show the stability factor you actually get in each barrel.
| Bullet | Length | Velocity | Twist needed | Sg at 1:7" | Sg at 1:8" | Sg at 1:9" | Sg at 1:12" |
|---|---|---|---|---|---|---|---|
| 40 gr varmint | 0.700" | 3550 fps | 1:10.3" | 3.25 | 2.49 | 1.97 | 1.11 |
| 55 gr FMJ | 0.755" | 3150 fps | 1:10.9" | 3.61 | 2.76 | 2.18 | 1.23 |
| 62 gr M855 | 0.906" | 2950 fps | 1:8.9" | 2.41 | 1.85 | 1.46 | 0.82 |
| 69 gr match | 0.906" | 2850 fps | 1:9.4" | 2.69 | 2.06 | 1.62 | 0.91 |
| 77 gr match | 0.985" | 2700 fps | 1:8.8" | 2.35 | 1.80 | 1.42 | 0.80 |
Green is properly stable (1.5 and above), amber is marginal, red will not stabilize. Figures are calculated with the Miller twist rule at standard sea-level conditions, the same way the mobile app does it. Velocity is listed for reference and is not part of the calculation.
Check your own AR-15 load
Pre-filled with a typical AR-15 bullet. Change the bullet dimensions or twist to match your rifle.
Bullet
Barrel
Conditions
Bullet length matters more than weight — measure it or take it from your load data rather than guessing.
Matches the app’s stable classification at standard atmosphere.
Twist recommendation
Working figures
How twist rate works in AR-15
A bullet in flight is trying to tumble. Air pressure on the nose creates a force that wants to flip it end over end, and the only thing preventing that is the spin the rifling gave it. Twist rate sets how much spin the bullet leaves with, so it sets which bullets your barrel can actually stabilize.
Length is the driver, not weight. Long, sleek, high-BC bullets are harder to stabilize than short stubby ones of the same mass, which is why two bullets of identical weight can have very different twist requirements. The Miller rule used here accounts for that directly, which is why every figure on this page is calculated from bullet length rather than looked up from weight alone.
Aim for a stability factor of 1.5 or better rather than merely clearing 1.0. The figures here use the mobile engine’s standard-atmosphere Sg formula. In the app, actual stability is corrected for temperature and station pressure: colder, denser air reduces stability.
Twist rate for other cartridges
Recommended twist, the factory rates you will actually find, and a calculated stability chart for the bullets each cartridge shoots.
Want the raw calculator without a cartridge preset? Use the twist rate calculator.
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