The rate of twist determines the optimum bullet weight/length for a given caliber and speed of the bullet by applying the proper spin on the bullet to prevent the bullet form yawing and pitching... Expressed in terms of the number of revolutions per inch of barrel length, this ratio is commonly expressed by designations such as 1:9, 1/9 or 1 in 9 twist, the 1 represents 1 twist, the 9 represents inches of barrel length... A good rule of thumb is that the heavier, or "longer" a bullet is, the faster the twist rate needs to be to stabilize it in flight, therefore a lighter "shorter" bullet needs a slower twist rate to give proper bullet spin for correct flight... Over stabilization of bullets can occur when larger heavier bullets in a given caliber are fired at very high velocities...
In general for the .224:
1/7 Good twist for the 50-100 grain bullets. Excellent accuracy with 60 grain bullets and up...
1/8 Good twist for the 50-80 grain bullets. Excellent accuracy with 55 grain bullets and up...
1/9 Good all around twist ratio. Will shoot well with bullets anywhere from 40-75 grains in weight...
1/10 Good twist for the 40-62 grain bullets, but favoring the lighter side a little more...
1/12 Good twist for the 40-55 grain bullets...
1/14 Good twist for the 40-45 grain bullets...
You can use the Greenhill formula to determine the proper bullet to twist ratio... It is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers...
This can be simplified to:
Twist = 150 X D2/L (D2 = D squared)
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula...