Rate of Twist


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WayBeau
February 1, 2012, 09:38 AM
Please correct me if I'm wrong, but when referring to the rate of twist of a rifle barrel, when they say 1:9, does that mean that the rifling has made one complete turn in nine inches? What are the advantages to have a lower rate of twist? Wouldn't a higher rate cause the bullet to spin faster helping to increase accuracy?

Sorry if this is a beat-to-death topic, but I've been trying to educate myself about rifle barrels lately and this is my latest question and I couldn't really find too much info in a search.

Thanks.

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browningguy
February 1, 2012, 09:48 AM
Yes, that is what it means. Faster twists are needed to stabilize longer (usually heavy for caliber) bullets. Too fast a twist however can lead to bullet fragmentation of too short (usually light) bullets. For example shooting 40-45 gr. varmint bullets in a 7 or 8 twist 22-250 may lead to the bullets coming apart when they exit the muzzle. Alower number means a faster twist, so a 1:9 is faster than a 1:12, when discussing faster or slower related to barrel twist this directly relates to the RPM of the bullet.

Generally speaking, for high accuracy you want the slowest twist that will stabilize the selected bullet. There is a lot of math and physics involved but many believe that a bullet can indeed be over-stabilized, although to be honest I don't understand all the math behind that statement so withhold jusdgement.

This leads to seeing guns such as a 1000 yard 6mm shooter using something like a 7.5-8 twist to stabilize the 107 SMK or 115 gr. VLD's, while a 6mm 100-200 yard benchrest shooter may select something like a 11 -14" twist just to perfectly stabilize a 69-80 gr. flat base bullet.

murf
February 1, 2012, 10:27 AM
mornin, waybeau.

yes, a one in nine inch twist means the rifling makes one complete revolution every nine inches (even if the barrel is only three inches long). also called "pitch" (like "thread pitch" of a machine screw or nut (1/4 - 20)). twist can be right or left handed (just like a machine screw can have right or left handed threads).

most people say "slower" (higher numbered rate), or "faster"(lower numbered rate) rate of twist. the high/low thing gets confusing. leading to your final question.

the higher the rate of twist, the lower the number in that rate's designation. a one in nine twist rate spins faster than a one in twelve rate. so, the lower the number IN THE DESIGNATION, the faster the spin.

the advantage of a lower rate of twist is accuracy. maybe. the advantages of a higher rate is accuracy. maybe. each bullet needs a certain amount of spin to stabilize correctly.

murf

Art Eatman
February 1, 2012, 11:14 AM
Heavier bullets in any given caliber are longer than the lighter bullets. Length is the determining factor in choosing a rate of twist.

As example, in .223, a 1:14 twist is sufficient for bullets up to 55 grains. Those who use the heavier bullets such as 70 or 80 grains commonly go to around 1:7.

In the .30-'06, a 1:10 twist stabilizes bullets in the range of 110 grains on up to at least 180 grains.

holdencm9
February 1, 2012, 04:02 PM
Yes, different caliber/weights will need different rates of twist. An under-stabilized bullet can keyhole into the target (imagine it wobbling as it flies...kind of like the way a football does when I throw it :o)

Allegedly, an over-stabilized bullet can spin so fast it tears itself apart. I have heard of this happening with .223 rounds less than 50 grains in a 1:7 twist. But in any case, it would have to be a very fast twist and very lightly-constructed bullet. I think for the most part, a given bullet has a wide range of twist that will adequately stabilize it.

You can figure out how fast a bullet is spinning by the barrel twist rate and the muzzle velocity. For instance, a bullet that travels 2800 feet per sec in a 1:7 twist barrel.

(2800 fps = 33,600 inches per second) divided by 7 inches per full rotation = 4800 rotations per second!

Try spinning a football that fast!

As you can see, the higher the number in the denominator , the slower it will rotate.

dagger dog
February 1, 2012, 06:21 PM
WayBeau, google Greenhill's formula, you can get more info including twist rate calculators. Just more food for the grey matter!

joed
February 1, 2012, 06:40 PM
You have the definition correct, but there is more to it than that. As said, to fast a twist and lighter bullets won't work, to slow and heavy bullets won't work.

If you search the internet you can find tables that will tell you what twist is needed for a given weight of bullet for different cartridges.

I used to own a Savage 10fp in .223 that had a 1:9 twist. It would shoot the 69 gr bullets extremely accurate. But 50 gr just didn't work well at all. Ended up selling the gun because I wanted to shoot light bullets for varmint hunting.

You also should know how to pick a bullet, there are a lot that don't understand this concept. You don't shoot a 69 gr match bullet in a .223 at varmints, the reason being ricochets. Varmint bullets are meant to disintegrate on contact.

There is a lot to know about rifles, you'll learn as you go along.

Hummer70
February 1, 2012, 07:43 PM
A 12 twist will stabilize a 30 cal 190 grain bullet to 1000 yards. I won the Palma Trophy Match twice with a 12 twist 30.06. When I order new barrels for 30 cal I get 12 twist. If I was going to shoot nothing heavier than 175 grain I would go 13 twist.
308 Bench rest shooters using 168 MK and lighter bullets went to 14 twist. I have a 13.8 twist Obermeyer that stabilized 168s to 1000 yards just fine.

Normally on 69 Sierras they call for 7-10 twist barrels for 223 ammo. I have a buddy who got a Savage Varmint 22-250 with 12 twist and it just loves 69 gr. Sierras. It shoots about an inch at 200 yards.

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