1/9 vs 1/8 twist

[Ron Padilla]

is there any real difference I have a 9 twist heavy stainless with a twist flute on the way, what sayeth use guys I wont be disappointed if I should of held out for a 8 twist it was what I could afford at the time. it is an 18 inch 1/9 twist heavy stainless with a twisted flute.

Thanks

[molonlabetn]

Only real difference (barrel quality aside) would be if you plan on shooting 75gr or heavier bullets. 1-9 will stabilize up to 69gr reliably, and even some 75-77gr in the right conditions, but I wouldn't count on it in a sub-20" barrel.

[Ron Padilla]

when you say 75 to 77 gr in the right conditions, can you explain that a little more in detail for me, however I have read that the 69 gr hollow point will drop a dear. I have been thinking about some pig hunting and not sure if the 69 gr will drop a sow. thanks for the info.

[molonlabetn]

when you say 75 to 77 gr in the right conditions, can you explain that a little more in detail for me, however I have read that the 69 gr hollow point will drop a dear. I have been thinking about some pig hunting and not sure if the 69 gr will drop a sow. thanks for the info.

In cold conditions cartridges do not develop as much pressure and velocity is reduced... bullet rpm is directly related to muzzle velocity and twist, and bullet rpm determines the gyroscopic force of the bullet to maintain orientation. Higher velocity requires less twist for a given moment of inertia (diameter & density of the bullet)... Also, cold temps and/or high humidity affect aerodynamics to adversely affect stability, sometimes enough to overcome the gyroscopic stability.

Also... Bullet construction and accuracy have more affect on effectiveness than sheer mass or velocity, in most cases. I doubt that a 77gr bullet would succeed where an equivalent 69gr failed. Edited May 19, 2014 by molonlabetn

[Dolomite_supafly]

Barrel length has zero to do with bullet stability.

[molonlabetn]

Barrel length has zero to do with bullet stability.

It does if the bullet does not achieve enough velocity (read rpms) to stabilize... Or maintain enough velocity at the range you are shooting.

Muzzle velocity at a given twist rate affects stability... Barrel length affects velocity. This can be tested easily by shooting a 77gr bullet through a 10" 1-9 twist barrel then a 20" 1-9 barrel at distance, the lower velocity projectile will destabilize first every time. Edited May 20, 2014 by molonlabetn

[Dolomite_supafly]

Velocity affects bullet stability but barrel length does not.

 

Barrel length affects velocity but barrel length has no direct affect on bullet stability.

 

I have had people tell me a bullet must make one complete revolution in the barrel to be stable. That is complete and utter BS.

[molonlabetn]

I have had people tell me a bullet must make one complete revolution in the barrel to be stable. That is complete and utter BS.

That's absolutely true... Im talking about physics and aerodynamics; muzzle velocity directly influences projectile rpm at a given twist, projectile rpm in turn determines how much gyroscopic force is available to counteract the aerodynamic forces. Small variances make a large difference... A bullet doesn't need to make a full rotation in the barrel as long as its exit rpm is sufficient to ensure stability, conversely, a bullet can make multiple rotations in a long barrel with insufficient twist to ensure stability.

Barrel length affects muzzle velocity. Period.

The rpm of a 77gr bullet from a 1-9 twist 10" barrel does not equal the rpm of a 77gr bullet from a 1-9 twist 20" barrel even though the twist is the same, rpm is determined by muzzle velocity. At some point the barrel length (and muzzle velocity) is low enough to not provide sufficient bullet rpm to fully stabilize for the duration of flight. Edited May 21, 2014 by molonlabetn

[molonlabetn]

Velocity affects bullet stability but barrel length does not.

Barrel length affects velocity but barrel length has no direct affect on bullet stability.

You just made two contradictory statements... For a given cartridge, velocity and barrel length are directly related.

[Steelharp]

Barrel length affects velocity, velocity affects stability, ergo, barrel length affects stability by velocity.

[Ron Padilla]

I did find more information by using of all things the interweb, here is one of the links to what I discovered www.6mmbr/223rem.html
the next is http:kwk.us/twist.html and the third one was www.snipercountry.comhottips/twistrate.htm. from what I read on one of those sites is that a 1/9 twist in a 18 inch barrel will handle up to a 70 gr round with out a problem. If you look at the sites and determine if I read it wrong let me know here. it is getting to late to work on the other equations but will be looking at them again tomorrow.

Thanks for the imput

[Dolomite_supafly]

If I have a 7.5" barrel and a 20" barrel and both have identical muzzle velocities is any one more stable than the other?

 

Now if I have two 20" barrels and one has a velocity substantially lower than the other are they both equally stable?

 

It is velocity that determines a bullets stability if all else remains the same. A bullet fired out of a 7.5" barrel will not be less stable than a 20" barrel if the velocities are the same. It is the velocity that determines the bullets stability not the barrel length. Now a longer barrel will increase the velocity but a longer barrel does not guarantee a stable bullet if the velocity is lower than the threshold.

[molonlabetn]

Why does anyone use a barrel longer than 7.5 inches then? If you can get the same muzzle velocity as a 20" barrel provides, from a 7.5" barrel with the same twist, using the same load, then yes the stability would be identical.

You and I both know that's not even close to realistic.

Using the same load, the barrel that spins the bullet to a higher rpm will be more stable, whether you arrive there by increasing velocity or twist. The only way to increase the velocity of a fixed load is to increase barrel length. Edited May 21, 2014 by molonlabetn

[molonlabetn]

Here's a GREAT calculator. If you know your muzzle velocity and twist rate, you can predict stability.


http://bulletin.accurateshooter.com/2014/05/are-you-spinning-your-bullets-fast-enough/

[molonlabetn]

Here's a GREAT calculator. If you know your muzzle velocity and twist rate, you can predict stability.


http://bulletin.accurateshooter.com/2014/05/are-you-spinning-your-bullets-fast-enough/

Using that calculator, anything below 2500fps using a 77gr SMK is in the realm of 'marginal stability' through a 1-9 twist, but will be stable down to 1250fps through a 1-8 twist.

To put it in perspective, you'd be hard pressed to get much above 2500fps using a 77gr bullet with much less than a 16" barrel, but with a 1-8 or better twist you could ensure stability even at 7.5" barrel velocities.

Meanwhile I'd like to hear about the 7.5" barrel that can push a 77gr SMK above 2500fps. Edited May 21, 2014 by molonlabetn

[Dolomite_supafly]

I am just saying stability is directly related to velocity, not barrel length. Now barrel length has an impact on velocity but not a direct link to stability.

[TnShooter83]

Why does anyone use a barrel longer than 7.5 inches then? If you can get the same muzzle velocity as a 20" barrel provides, from a 7.5" barrel with the same twist, using the same load, then yes the stability would be identical.

You and I both know that's not even close to realistic.

Using the same load, the barrel that spins the bullet to a higher rpm will be more stable, whether you arrive there by increasing velocity or twist. The only way to increase the velocity of a fixed load is to increase barrel length.

 

Depends on the bullet we are using?

Are we using light jacketed 40gr Varmint bullets at 3400 fps in a 1/8 or 1/7 barrel?

If so, you better back off on the RPM. 

 

I'm not trying to prove you wrong. Or start an argument

I'm stating facts form personal experience. 

I don't think anything that breaks apart during flight is "stable"

 

Unless I think I'll ever use 72+gr bullets. I feel 1/9 is a good all around twist.

Edited May 21, 2014 by TnShooter83

[molonlabetn]

Depends on the bullet we are using?
Are we using light jacketed 40gr Varmint bullets at 3400 fps in a 1/8 or 1/7 barrel?
If so, you better back off on the RPM.

I'm not trying to prove you wrong. Or start an argument
I'm stating facts form personal experience.

Bullet structural integrity is a completely different issue altogether.

But yes, every bullet has a performance envelope... With sectional density and mass at the lower end, determining how fast the bullet must spin to maintain stability, limited by structural integrity at the other end of the spectrum.

[molonlabetn]

I am just saying stability is directly related to velocity, not barrel length. Now barrel length has an impact on velocity but not a direct link to stability.

I can agree to disagree about the semantics, direct or indirect... My only point to the OP was that barrel length matters if a bullet must accelerate enough to stabilize in a given twist, there's no way around it. There is plenty of data out there for velocity and barrel length correlation with any cartridge... So whether the chicken or the egg came first, it is undeniably possible to choose too short of a barrel in a given twist to get the required velocity for a bullet to stabilize. For a 77gr bullet, the magic velocity is around 2500fps at a 1-9 twist, which you won't get out of a barrel much shorter than 16" without going to a hot load like DoubleTap or MK262.

Thank you for making me think about the problem, either way. :-) Edited May 21, 2014 by molonlabetn

[Ron Padilla]

So from the above, it seems safe to say that an 18 inch barrel is going to create more velocity than a 16 inch barrel, but less than a 20 inch barrel. But if an 18 inch barrel creates a higher velocity it should hold true that it would spin a higher grain round equally to a tighter twisted but shorter barrel. Or is that not a safe assumption, and the lower twisted barrel even if longer will still not propel the heavier projectile and still need the tighter twist to propel the projectile accurately?

[molonlabetn]

So from the above, it seems safe to say that an 18 inch barrel is going to create more velocity than a 16 inch barrel, but less than a 20 inch barrel. But if an 18 inch barrel creates a higher velocity it should hold true that it would spin a higher grain round equally to a tighter twisted but shorter barrel. Or is that not a safe assumption, and the lower twisted barrel even if longer will still not propel the heavier projectile and still need the tighter twist to propel the projectile accurately?

Yes.

If a 20" 1-9 twist barrel develops, say, 2800fps with a 77gr bullet, it will be spinning at roughly 224000rpm... To get the same rpm, with the same cartridge at the muzzle velocity produced by a 10" barrel, say 2200fps, you would need almost exactly a 1-7 twist.

Another part of the equation though, is that at lower velocities, less rpm is needed to counteract aerodynamic forces... But the relationship is not linear. Edited May 21, 2014 by molonlabetn

[molonlabetn]

Short answer... 1-9 will work with 77gr if your barrel can push it fast enough, but 1-8 will work for sure at any velocity.

I have a 20" 1-9 .223 bolt action that loves 75-77gr almost as much as 68-69gr.