Rifling Twist Rates

[CaliberFiveSeven]

I realize that barrel rifling is for gyroscopic bullet stabilization but what are the advantages / disadvantages effects of various rifle barrel twist rates such as 1 in 7 , 1 in 8, or 1 in 11 on a bullet?

How is the bullet weight affected by twist rate?

Any advantage whether a RH or LH?

 

[blackDdefense]

The twist is designed to spin stabilize the bullet. IT depends on bullet size and shape. Certain weights need a higher RPM in order to overcome the natural tendency of the bullet wanting to go "ass over apple cart" if you catch my drift. The military uses the 1/7 twist rate because of the size and shape of the M855 bullet. A lot of AR manufacturers use 1/8 or 1/9 to provide a wider variety of ammunition to the civilian shooter. In the case of the 5.56mm projectile ...the higher the weight...the faster the twist. That statement is very broad because it depends also on what "type" of bullet used. Confused yet?

 

[Iramo94]

Next question: Why can dangerous game rounds, sometimes 400 grains or more, get away with slow twists? Why aren't .45-70's shooting through 1-in-5 barrels?

 

[68wj]

... The military uses the 1/7 twist rate because of the size and shape of the M855 bullet...

Tracers are the main concern.

Bullet weight is determined by its material makeup and length (diameter is fixed to the respective caliber). Steel core (M855) and copper bullets are lighter than lead and tend to be longer for a given weight, thus needing more spin. With the .mil, tracers are even longer and must be spun faster.

 

[68wj]

Next question: Why can dangerous game rounds, sometimes 400 grains or more, get away with slow twists? Why aren't .45-70's shooting through 1-in-5 barrels?

This is the old standard and mostly accurate. It is based on traditional bullet construction and estimated bullet length for a given weight. Newer formulas have since been developed, but this is a good visual representation:

 

[Iramo94]

Wj, thanks for the info, really interesting. But that chart spawns one more question. Who in their right mind shoves a 460 grain bullet into a .308 cartrige?!

 

[68wj]

Wj, thanks for the info, really interesting. But that chart spawns one more question. Who in their right mind shoves a 460 grain bullet into a .308 cartrige?!

It isn't necessarily a .308 Win cartridge, but any .308 bored rifle. However, much of the chart may just expand the information into the theoretical realm. Unless, of course, you count the .3000 Rhino Express Magnum.

 

[MistWolf]

The confusion lies in the erroneous assumption a tighter twist (higher RPM) is needed to stabilize a heavier bullet. A tighter twist is needed to stabilize a longer bullet that has more weight bias towards it's rear, such as a spitzer. Large caliber bullets even in the 400-500 gr range are short & stubby.

Regarding the 223/5.56, the 1/9 twist does NOT allow use of a wider range of bullet lengths. Tests done by Sierra show that their 69gr SMK has more wobble when fired from a barrel with a 1/9 twist than it does from a 1/7 or 1/8

 

[Zak Smith]

The rotational inertia must be able to overcome the transverse force that is trying to push the bullet's tip. The mass distribution as a function of radius has a lot to do with the former, while length is the primary factor of the latter.

 

[MtnCreek]

^^^That's what I was going to say!

 

[CraigC]

The confusion lies in the erroneous assumption a tighter twist (higher RPM) is needed to stabilize a heavier bullet. A tighter twist is needed to stabilize a longer bullet that has more weight bias towards it's rear, such as a spitzer. Large caliber bullets even in the 400-500 gr range are short & stubby.

Exactly! It's the length, not the weight. Weight, or more specifically sectional density (ratio of diameter to weight), is just an indication of a longer bullet.

 

[Effigy]

So this begs the question, what's the disadvantage to opting for faster twist rate if it's able to stabilize heavier bullets?

 

[Ol` Joe]

[QUOTESo this begs the question, what's the disadvantage to opting for faster twist rate if it's able to stabilize heavier bullets? ][/QUOTE]

Over stabilization.
At long(er) range as the bullet slows down conciderably it can start to ride nose up. This affects the BC of the bullet and that changes the wind resistance, and drop of the bullet vs a properly stabilized one. It also makes it act up as the bullet crosses the sound barrier

 

[Funshooter45]

If you spin them too fast they might come apart in flight if the jackets aren't tough enough. Also, I have heard that faster twist slows the bullets down just a bit, so muzzle velocity might suffer.

 

[TonyAngel]

And to top all of this off, there's just no way to determine what a barrel is going to shoot the best without actually shooting it. I have one particular 1:7 twist barrel that shoots most accurately with 52gr HPs and a 1:9 twist barrel that shoots its best with 69gr bullets. Go figure.

If you want to wind up scratching your head, go over to Berger Bullets' website and check out the minimum twist rates on their bullets. Hardly any of them require a fast twist rate. If I remember, they only require a 1:7 twist for their heavier bullets for the .223.

 

[MistWolf]

A bullet cannot be over stabilized. It's either stable or it's not. What too much RPM does is affect tractability, that is, the nose will not tip over and follow the flight path. Tractability is only a concern to rounds fired at a high arc, such as artillery or long range precision shooting.

The disadvantages of a tight twist are increased wear of the throat and a loss of velocity. Both are very minor concerns in this case. TANSTAAFL- There Ain't No Such Thing As A Free Lunch