Barrel Twist Rate

[17remhunter]

My friend shoots a .280 AI. Cooper. He said he does not know the twist rate. He is shooting 168 Gr LRX bullets. I believe those bullets need a 1x8 twist however he said that the barrel twist rate should not make that big of a difference If his barrel is a 1x9. Will that affect his accuracy?

 

[Huntaddict]

Should be fine. 1:9 is fine for most bullets up to about 175. After that 1:8 would likely be optimal.

 

[270 Terminator]

In 55 yrs of doing this I found that your reloading press and the range are the only thing that can truly tell you what your rifle will and will not shoot

 

[lancetkenyon]

This is the 168 LRX mono. 1.587" long. Barnes recommended minimum twist is 1:8". Berger stability calculator says 1.42 SG "marginally stable" @ 4000' & 59°F.

The Cooper is a 1:9".

Buy a box and load them up. See how they shoot at your hunting elevation and temps. That is the only way to know. They shot OK in a 1:9" 7RM @ 3050fps for me.

Now, if you would have asked about the new.160 LRX, I would have told you no way. They DEFINITELY need the 1:8" twist.

 

[FEENIX]

My friend shoots a .280 AI. Cooper. He said he does not know the twist rate. He is shooting 168 Gr LRX bullets. I believe those bullets need a 1x8 twist however he said that the barrel twist rate should not make that big of a difference If his barrel is a 1x9. Will that affect his accuracy?

When in doubt, go directly to the source or do this ...

 

[LuvDog]

In the long run, yes. But it depends on a few things.

Berger's twist rate calculator says he needs more twist - especially if he's not at a higher altitude.

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

 

[akmtnhnt]

This is the 168 LRX mono. 1.587" long. Barnes recommended minimum twist is 1:8". Berger stability calculator says 1.42 SG "marginally stable" @ 4000' & 59°F.

The Cooper is a 1:9".

Buy a box and load them up. See how they shoot at your hunting elevation and temps. That is the only way to know. They shot OK in a 1:9" 7RM @ 3050fps for me.

Now, if you would have asked about the new.160 LRX, I would have told you no way. They DEFINITELY need the 1:8" twist.

In the long run, yes. But it depends on a few things.

Berger's twist rate calculator says he needs more twist - especially if he's not at a higher altitude.

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

Berger's stability calculator is not valid for any tipped bullets.

Use one that accounts for tips(density change at the front of the bullet). The 168lrx looks plenty stable in a 1:9 at sea level @ 2800fps if you account for the plastic tip. SG=1.69

 

[akmtnhnt]

Should be fine. 1:9 is fine for most bullets up to about 175. After that 1:8 would likely be optimal.

Stability has to do with length and density. Not really just weight. This is especially applicable since we're talking a copper bullet here.
But it's not valid so say a 1:9 will stabilize up to a 175. You can design a lead bullet at 175gr that absolutely will not stabilize in a 9. You can design a copper bullet at 150gr that won't stabilize in a 9 twist. You can design one that will. Weight doesn't tell you enough to know.

 

[akmtnhnt]

In 55 yrs of doing this I found that your reloading press and the range are the only thing that can truly tell you what your rifle will and will not shoot

Putting circular holes in paper doesn't tell you if a bullet is stable. Tumbling tells you it's not stable, but not tumbling doesn't mean it is stable.
You'd have to shoot enough to get a trued BC at distance and see what the degradation from the yaw is. Then there's the decreased terminal performance that paper can't tell you, and can be hard to quantify in the extremely limited sample size of most people's hunting.

 

[Huntaddict]

Stability has to do with length and density. Not really just weight. This is especially applicable since we're talking a copper bullet here.
But it's not valid so say a 1:9 will stabilize up to a 175. You can design a lead bullet at 175gr that absolutely will not stabilize in a 9. You can design a copper bullet at 150gr that won't stabilize in a 9 twist. You can design one that will. Weight doesn't tell you enough to know.

Of course it is valid - I agree with your thoughts however by stating a modern made bullet weight in a specific caliber, the length, weight, and density are already factored in. Then a quick stability calculator data check to ensure we are above 1.5 and we are done. Even copper bullets are subject to this rule

Easy button - sometimes the best solution

 

[Huntaddict]

Putting circular holes in paper doesn't tell you if a bullet is stable. Tumbling tells you it's not stable, but not tumbling doesn't mean it is stable.
You'd have to shoot enough to get a trued BC at distance and see what the degradation from the yaw is. Then there's the decreased terminal performance that paper can't

Question - explain the second part...if tumbling tells you its not stable, how does NOT tumbling say it is NOT stable? Isn't that the definition of stable - not tumbling?

Mono bullets can act differently and sometime do require a bit more twist however if your mechanicals are correct, the bullet should not tumble until it reaches trans/subsonic speeds. 1.46 is so close to 1.5 you will probably be fine

MOST tumbling from flight occurs when the bullet is approaching trans/sub sonic velocities however; If a muzzle is not properly machined, if the bullet has imperfections, or if there are other mechanical issues such as an imperfect chamber and seating, there are cases when a bullet can tumble right out of the muzzle. While these are much harder to detect, even in a barrel that is under 1.5 stability, generally if the mechanicals are good, a bullet wont tumble until it is reaching sub sonic velocities.


Real world - Take a 7mm PRC with a 180 grain VLD bullet and an 8 twist barrel at 3000 fps. It has a stability factor of 1.77 which is fine. If the barrel and components are all prepared correctly, tumbling would not occur until that bullet ELR is not properly stabilized well past 1500 yards.

JBM - Calculations - Stability

www.jbmballistics.com

Now if we take that same scenario with a 9 twist, we are at 1.4 - ragged edge of stability. However lets change it to a 175 class bullet and we are at 1.56 - stable.


Bullets have both a Center of Gravity and a center of pressure or inertia - as long as they are spinning consistently on the same axis - e.g. gravity or some other force has not altered their alignment, the bullet will remain stable and fly consistently.

 

[Huntaddict]

This is the 168 LRX mono. 1.587" long. Barnes recommended minimum twist is 1:8". Berger stability calculator says 1.42 SG "marginally stable" @ 4000' & 59°F.

The Cooper is a 1:9".

Buy a box and load them up. See how they shoot at your hunting elevation and temps. That is the only way to know. They shot OK in a 1:9" 7RM @ 3050fps for me.

Now, if you would have asked about the new.160 LRX, I would have told you no way. They DEFINITELY need the 1:8" twist.

1.42 is so close to 1.5 you will probably be fine - sometimes mono bullets do require more twist however the stability calculators are less about muzzle and more about down range stability. If there is an issue at the muzzle or even at a 100 yard target, it is likely something in the rifle or components of the bullet.

 

[dogz]

My friend shoots a .280 AI. Cooper. He said he does not know the twist rate. He is shooting 168 Gr LRX bullets. I believe those bullets need a 1x8 twist however he said that the barrel twist rate should not make that big of a difference If his barrel is a 1x9. Will that affect his accuracy?

Sounds like he's already shooting that combo right? So, how's the load actually shooting for him?

 

[VinceMule]

In one 7 Rem mag with a 9T, the 145 LRX proved to be very easy to tune, fast, and super accurate with a max load of IMR 4350 and R#19, 3250 fps, and 3300 respectively....just an option.