An excerpt from the FSD Blog, “Let’s Twist Again”:
In layman’s terms, the twist rate is the number of revolutions the rifling makes inside the barrel.
Think of a Slinky (younger folks, look it up). The spiraling of the toy is what it’d look like if you took away the outer metal of the barrel and only left the lands and grooves inside. You can stretch it out so that the spirals appear farther apart, or mash it together and the spirals will compact. The spirals in a Slinky are the revolutions we’re talking about.
The more times the spiral makes a full revolution, the lower the twist rate. For example, a 1:9 (to be read “one in nine”) twist rate means that, for every 9 inches, the rifling makes one complete revolution inside the barrel.
When it all comes down to it, we’re really talking about stabilizing bullets. That is, taking into account the air that the bullet must pass through and figuring out how long the bullet will remain stable.
The numbers of revolutions made inside a barrel determine the stability of the bullet. So, the lower the number, the more (and sometimes faster) the bullet is spinning, the more stable the bullet is.