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?
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
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.
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.(Sold right here at The Sportsman's Guide http://www.sportsmansguide.com/net/cb/cb.aspx?a=892641
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. A gunsmith should be able to do the job for between $350
and $700, barrel included.
Shop The Sportsman's Guide great selection of Rifle Ammunition!
Ron Spomer has been photographing and writing
about the outdoors for nearly four decades. He's written seven books, hunted on
six continents and been published in more than 120 magazines. He's currently rifles'
editor at "Sporting Classics," Travel columnist at "Sports Afield," Field
Editor at "American Hunter" and "Guns & Ammo" -- Optics Columnist at "North American
Hunter," Contributing Editor at "Successful Hunter," Senior Writer at "Gun
Hunter," and TV host of "Winchester World of Whitetail." He will write on Shooting Tips weekly for sportsmansguide.com. You can read his
blogs and catch some of his YouTube videos at www.Ronspomeroutdoors.com.