Long, long ago, back in the days of muskets, barrels were smooth. Due to the high cost and great difficulty of precision manufacturing, and the need to load readily from the muzzle, the musket ball was a loose fit in the barrel. Consequently on firing the ball bounced off the sides of the barrel when fired and the final direction on leaving the muzzle was unpredictable.
RIfling was invented in the 1400′s but but not come into common use until the 1800′s.
Rifling is described by its twist rate, which indicates the distance the bullet must travel to complete one full revolution, such as “1 turn in 10 inches” (1:10). A shorter distance indicates a “faster” twist, meaning that for a given velocity the projectile will be rotating at a higher spin rate.
Twist rate and the projectile’s length, shape, and weight al impact accuracy. In this article however, I’m going to ignore projectile length and shape and concentrate on barrel twist vs bullet weight.
In general, heavier bullets need a higher twist rate to get them spinning fast enough to stabilize them.
To put it another way, once you know your barrel’s twist rate, you can determine the recommended maximum weight of bullet.
Brownells.com has a nice chart on their website website, For 223 and 308 here are the numbers:
|1:9||For bullets heavier than 63 gr.|
|1:12||For bullets up to 63 gr.|
|1:14||For bullets up to 55 gr.|
|1:15||For bullets up to 55 gr. driven 4,100 fps or more|
|1:16||For bullets up to 55 gr. driven 4,300 fps or more|
|1:8||For bullets heavier than 220 gr.|
|1:10||For bullets up to 220 gr.|
|1:12||For bullets up to 170 gr.|
|1:14||For bullets up to 168 gr.|
|1:15||For bullets up to 150 gr.|
My .223 caliber C93 has a 1:12 twist. That means that I need to use bullets weighing 63gr. or lighter.
Measuring Twist Rate
Measuring the twist rate of your barrel is easy. Use your cleaning rod. Shove it down the barrel until the other end comes out of the breach. Secure a tight-fitting patch patch onto it. Place a piece of scotch tape or masking tape around the cleaning right rod at the spot it exits the far end of the barrel. Place a dot or mark on the tape.
Now slowly start pulling the cleaning rod out of the barrel while watching the mark you made on the tape. When the mark has made exactly one revolution stop. Measure the distance from the end of the barrel to the tape. If the distance is 12 inches then you have a 1:12 twist rate.