Rifle Twist ratio- pros and cons

[Trailhawk]

Can someone explain the pros and cons of rifle twist. I get the more twist the more bullet rotations, but whats the best ration and advantages/disadvantages?

[Remek]

Easy enough.

The rotations cause the bullet to spin like a top, or gyrometer. It stabilizes the bullet in flight.

More twist, more stability.

Some (the US military) argue that the twist should be just high enough to stay stable in flight, but then tumble when it hits something, causing more damage than the bullet could manage when going straight through. I agree you can make a bullet do better damage this way, but some people think it does not make that much difference.

If you look at competition rifles, their twist ratio will be higher (more twist/inch) and therefore, the bullet runs more true. You can imagine if that spinning bullet is pushed just to the side a bit, it will change the trajectory by the force of the relative wind as the bullet flies through the air.

(edit: I forgot to mention, higher twist requires more energy, and therefore, the bullet may spend more time in the barrel and exit a slower velocity.)

There are plenty of discussions on the net as to the best twist for a given caliber, and a net search should put you into what you want quickly.

[FiremanBob]

One reason that competition rifles (at least the 5.56 ARs) have higher twist rates of 1:7 rather than the 1:9 typically found on civilian ARs is that they shoot heavier bullets - 77gr grain instead of 55 or 62. For a given caliber you need a faster twist to stabilize the heavier bullet. Match rifles use the heavier bullet because it has a higher ballistic coefficient so it is less affected by wind and drag over long ranges.

[bryanrheem]

I personally prefer 1:7 twist ... and I shoot everything from 55 up w/o a problem.

BUT, I am not a competition/target shooter.

[jdonovan]

Can someone explain the pros and cons of rifle twist.

bullet stability.

whats the best ration and advantages/disadvantages?

depends on your use.

Light for caliber bullets usually use a slower twist. 1:12
heavy use a higher twist 1:8

Very lightly built bulles at high speed can be torn apart in high twist guns. Run a .224 40gr varment bullet out of a 1:7 barrel at 3500 FPS and you'll have a shotgun by 20 yds.

Conversely run a 240 gr .308 @1000 FPS in a 1:14 and you'll be lucky to be on paper consistently at 50 yds.

generally for a given caliber there are generally accepted 'good' rates of twist. Unless you're doing something unusual those twist rates will generally work just fine.

[Taggure]

Just remember that the amount of twist on the bullet is also dependent on the length of the barrel

1:10 twist with a 20" barrel = 2 revolutions
1:10 twist with a 25" barrel = 2.5 revolutions

1:9 twist with a 18" barrel = 2 revolutions

and so forth.

[jdonovan]

Just remember that the amount of twist on the bullet is also dependent on the length of the barrel

yes, but what you are implying is not correct.

A bullet will take to the rifling in the first inch of barrel and will then be spinning at the rate of twist of the barrel. Anything else would have the bullet spinning at less than the rate of twist of the barrel which would lead to MASSIVE jacket fouling, significant gas leakage and other things that just don't happen with properly sized and constructed bullets.

[Remek]

If you have some patience and interest, this will provide a great base for understanding ballistics as it relates to rifling, both during barrel passage and afterward.

http://library.med.utah.edu/WebPath/TUT ... NBLST.html

[gunderwood]

Why do you ask?

The simple answer is that bullets are spin stabilized vice fin stabilized or some sort of active arrangement. If a projectile is not stable, it won't, by definition, fly point forward into the oncoming air flow. It should be obvious, but to build an accurate projectile system you must have the projectile stabilized. There is a well known estimate of spin stability which is called the Miller Twist Rule (http://en.wikipedia.org/wiki/Miller_twist_rule) which attempts to make a complex phenomena into a simple formula with easily measured parameters.

There is a wealth of knowledge beyond that (which is why I'm asking why you're asking), but that's the point. Stabilize the projectile and the bullet will fly point forward and be accurate. There are negative side affects to spinning a bullet, which is why we have a bunch of twist rates even within the same caliber. Once stabilized, there is no point to spinning it faster as that only exacerbates the negatives of spin stabilization. For most shooters, it's a non issue as most firearms on the market are sold with the proper twist rate for common weight bullets. Only those shooting well outside the norm or going for extreme accuracy care otherwise.

[M1A4ME]

Not enough spin (barrel twist rate too slow for the length of the bullet) means an unstable bullet that usually gets so unstable so fast it won't hit the target at longer ranges. I've seen 10" groups at 25 meters from an AR15 with a 1 in 12" twist rate barrel when the owner was shooting 62 grain military surplus through it.

Did you know that Remington and Winchester came out with similar cartridges back in 1955? The .243 Winchester, still a favorite cartridge for deer hunting and long range varmint hunting had a barrel twist rate of 1 turn in 10". Remington's very similar offering was call the .244 Remington and it could actually push the light weight varmint bullets a little faster than the .243 Win. However, Remington gave their .244's a barrel twist rate of 1 turn in 12". Slower than the Winchester rifle/cartridge. Both used 6MM bullets but the Winchester, with the faster twist rate, could use heavier bullets that were better for deer (the faster twist rate stabilized heavier bullets as well as the lighter varmint bullets). The .243 Win. is still here today. The .244 Rem. faded away. After awhile Remington changed the twist rate on their barrels to make them stabilize heavier bullets and renamed the cartridge "6MM Remington". It is still not as popular as the .243 Win. partly because by the time Remington woke up and fixed the problem they were way behind the .243's years of popularity and use out in the field.

Some people say barrels that spin the bullet too fast can result in less accuracy than heavier bullets. Some of us don't shoot well enough to notice .

Every hear/read someone recommend a dedicated .22 upper for your AR15 vs. a .22 conversion kit? Its because of the difference in barrel twist rate between .22's and .223's. The .223 caliber rifles have a faster twist rate and some people report that it degrades the accuracy potential of the rifle when .22 ammo is fired through the 1 turn in 9" .223 barrel. I say it depends on the rifle and the ammo. I have an AR15 with a 1 turn in 9" barrel that is a tack driver at 25 meters with the right brand of .22 ammo.

You need to experiment with different brands/bullet weights in a rifle. Two identical rifles may shoot better groups with different brands or bullet weights.

Having said that, if you plan to shoot heavier (longer) bullets in your rifle, you'll want a faster rate of twist. It'll still shoot the lighter bullets, too.

Oh, if you plan to reload be aware some of the varmint bullets are made with thinner jackets to improve expansion at lower velocities (not every .224 diameter bullet is shot through a .223 or even higher velocity varmint rifle.) Shooting those through a faster twist rate barrel can result in the bullets tearing themselves apart before it gets to the target. They leave a pretty smoke trail through the air behind them and you'll have less than 5 holes in the paper when you shoot your 5 shot group. BTDT Hornady SX 45 grain spire points (.223 Rem.) through my Mini14 (1 turn in 9") instead of my Rem. M700 (1 turn in 12").

[Taggure]

Just remember that the amount of twist on the bullet is also dependent on the length of the barrel

yes, but what you are implying is not correct.

A bullet will take to the rifling in the first inch of barrel and will then be spinning at the rate of twist of the barrel. Anything else would have the bullet spinning at less than the rate of twist of the barrel which would lead to MASSIVE jacket fouling, significant gas leakage and other things that just don't happen with properly sized and constructed bullets.

Thanks for the correction I was just trying to simplify it.

[cpttango30]

Twist all gets determined my length of bullet not weight as many believe. Speed also has a factor in it as well.

22-250 can shoot 55gr with a 1:14 twist while a 223/556 needs a 1:12 twist.

A 1:7 twist will cause some lighter 224 bullets to just spin apart. Put a 40gr V-max in a 223 and load it a little on the upper end and POOF they will just blow apart.

Too fast no good to slow no good either.

You can also get barrels with a progressive twist for some ULTRA overboar cartridges like the 22-6mmAI. What happens is the twist at the throat is maybe a 1:14 then it progresses to a 1:7 twist by the end of the barrel. I know of one guy that has one and it is on a 22-6mmAI and the barrel is something like 34" long.

For AR a 1:8 or 1:7 twist will allow you to shoot everything from 50 to 80gr (Depending on bullet manufacture).