Twist Rate
How fast the rifling spins a bullet, expressed as one turn in N inches (e.g., 1:7) — it must match bullet weight for stable flight.
Twist rate describes how many inches of barrel it takes for the rifling to spin the bullet one full revolution. A '1:7' twist completes one rotation every 7 inches and is 'faster' than a '1:9'. That spin gyroscopically stabilizes the bullet in flight, keeping it point-forward instead of tumbling.
The key is matching twist to bullet: heavier, longer bullets need a faster twist to stabilize, while very light bullets can over-stabilize in a fast twist. For the AR-15, a 1:7 or 1:8 twist handles heavy 69–77gr match bullets, while 1:9 suits lighter 55gr loads. Use a twist-rate calculator (Miller stability) to confirm a given bullet will stabilize in your barrel.