So is that why my .17 Rem has a 1:9" twist and my .45 ACP has a 1:18.5" twist?
No.
The rate of twist is based on the size of the bullet not it's weight.
It is the ratio of bullet length to bullet diameter.
However, for a given diameter, a heavier bullet is usually longer than a lighter bullet.
In 1879 Sir Alfred George Greenhill developed the Greenhill Formula for Rifling Twist to determine necessary rates of twist for artillery pieces.
Greenhill’s original formula originally stated;
The twist required in calibers, equals 150 divided by the length of the bullet in calibers.
(Expressing projectile length in Calibers is an old artillery practice.)
Today we express bullet length in inches or milimeters.
Another problem with Greenhill’s original formula was that it was designed around heavy, round projectiles moving within a small window of velocities.
The value of 150 used by Greenhill was based on spherical, pure lead projectiles launched with black powder.
So in order to take in account modern bullet designs and small arms bore diameters a modified Greenhill formula is used.
The modified (simpler) formula is;
T = Q × D²÷L
(Twist equals Velocity Value times Bullet Diameter squared, divided by Bullet Length)
Where:
D = bullet diameter in inches
L= bullet length in inches
Q = a range of velocity “value”
T = rate of rifling Twist
Values for velocity ranges are;
Below 1000fps - 45
1000-1499fps - 125
1500-2999fps - 150
3000-4500fps - 180
Now this is all well and good and for the most part will get you into the ball park but it doesn't take into consideration the wider range of velocities obtainable with today's propellants nor does it allow for the various alloys and materials used in modern bullet construction.
So, we come to the Sierra Bullet Company formula.
Actually it's formulae, since Sierra uses a different formula each for rifles and handguns and they are based on velocity whereas the original Greenhill formula was based on specific gravity.
The Sierra formula for Rifles is T = 0.06 × V × D² ÷ L
The Sierra formula for Handguns is T = 0.05 × V × D² ÷ L
Where:
D = bullet diameter in inches
L = bullet length in inches
V = velocity in feet per second
T = rate of rifling Twist
So you see the two disiplines seldom agree but each one will get you close to the twist needed for reliable stabilization.
I prefer the Sierra method because you can calculate the necessary rate of twist based on the down range velocity of the bullet.
It will tell you at what range the bullet is likely to become unstable.
All of the above helps illustrate why, when firing a heavier and or slower bullet you need a faster rate of spin for stability.
And why a 180gr Barnes all copper bullet can use a slower rate of twist than a 180gr pure lead bullet.
Or why a .454 Casull firing a hard cast SWC needs a faster twist than a Colt 1860 Army firing a pure lead round ball.