We all understand that a rifle barrel has spiral grooves (rifling) in order to impart spin to bullets. This stabilizes them so they fly true, like a well-thrown football. But what's all this talk about twist rates like 10-inch or 1-in 14? What's that all about and how important is it?
The standard 10- or 12-inch twist barrel of a .30-06 will stabilize
all these 308 caliber bullets, which run from 100- to 165-grains, but a 220-grain one
would probably require an 8-inch twist barrel to stabilize.
Twist rate numbers reference how many inches of barrel are used to make one complete, 360 turn of the rifling lands/grooves. This is important because the longer the bullet, the faster the twist must be to stabilize it. So, a barrel with 1 rifling turn in 12 inches would stabilize a longer bullet than one with 1 turn in 20 inches. How long of a bullet? That depends on the caliber (diameter) of the bullet and its velocity.
Fortunately for us, engineers have figured out optimum twist rates for standard cartridges using typical bullet weights. Manufacturers build barrels with appropriate twist rates to match based on industry standards.
Select manufacturers may change these slightly, but rarely radically. Some cartridges, such as the .30-06, accommodate an amazing number of bullet weights/lengths. With other cartridges, a smaller variety of bullet weights/lengths are stabilized. For instance, most .30-06 barrels have a 10-inch or 12-inch twist. These are adequate to stabilize bullets as light (short) as 100 grains and as heavy (long) as 200 grains, sometimes 220 grains. A safer upper end (long bullet) is 200 grains.
Length is the big factor here rather than weight. A 220-grain, lead flat-nosed bullet might stabilize in a 10-inch twist .30-06, while a 220-grain boat-tail spire point would be too long to stabilize.
In the .22-250 Rem., the 12-inch twist rate usually stabilizes lead-core bullets from 40 grains to 55 grains, sometimes 60 grains.
Berger makes many VLD bullets, which are extremely long and ballistically efficient,
but if they get too long, faster-than-traditional barrel twists are needed to stabilize them.
Note the twist recommendations on the boxes.
Same caliber bullets come in a wide variety of weights/lengths.
A single barrel twist rate can't stabilize them all. The longer the bullet,
the faster the twist needs to be to stabilize it.
Bullets made of materials with less specific gravity than lead (less mass per given area,) such as copper or gilding metal, need to be longer for a given weight. But if too long, they won't stabilize in traditional twist barrels. This is why the new non-toxic varmint bullets from Nosler, Barnes and Hornady are so light. In .22 caliber, a 40-grain Nosler BT Lead-Free bullet is almost as long as the lead-core 50-grain Ballistic Tip. Nosler specifies that the 40-gr. BT Lead-Free requires a 1-12-inch twist barrel. Barnes specifies faster barrel twist rates for many of its longer bullets.
So why bother with the heavier bullets? Why not just stick with shorter, lighter bullets that stabilize in slower twist barrels? Because short bullets have lower ballistic coefficients than long bullets. Lighter weight reduces ballistic coefficient, too. The upshot is that a light, short bullet that stabilizes in standard twist barrels suffers decreased ballistic performance. It drops faster than longer, heavier bullets and loses more energy at all ranges.
The cure for this is indeed additional bullet length and the weight that comes with it, but such bullets require faster rifling twists. And those are slowly becoming more common. I suspect, and hope, that this will accelerate because there are so many long bullets coming on line.
The new VLD (Very Low Drag) bullets from Berger and others are extremely long, sleek, boat-tail designs that deliver optimum long range performance. But when you try shooting them in traditional twist barrels, they don't stabilize. This is why you rarely, if ever, find factory loaded ammunition with extreme VLD bullets. VLD bullets are instead sold as components to handloaders with warnings printed on the box about necessary twist rates.
For the most part, you don't have to worry about twist rates if you shoot factory loads and factory standard rifles. Both are designed and built to work well together. But if you get into non-toxic ammunition or VLD bullets, you could begin seeing your accuracy suffer.
If your groups begin opening, suspect poor stabilization. If bullet holes in targets begin elongating, your bullets aren't stabilizing. They're wobbling and hitting the target slightly tilted on their sides. Horribly unstable bullets will keyhole (enter sideways). Higher velocity sometimes cures this, but that risks dangerous pressures. A safer, better solution is to buy a fast twist, aftermarket barrel.