Barrel length and twist rate

[Coyote Shadow Tracker]

ORGINAL QUESTION:

Let's see if I can ask this question right.....
How does barrel length impact the stabilization of a bullet? Obviously a 16" barrel has 8" less influence on the bullet than a 24", same twist rate
Lets use a 105gn bullet out of a 243. 1/9 twist If I can't get it stabilized from a 16" barrel would a 20" barrel work because it was influenced by 4 more inches of rifling?

TWIST RATE EFFECTS STABILIZATION (there are many mathematical programs to prove this).
CHANGING THE LENGTH OF THE BARREL CAN EFFECT THE"VELOICTY" OF THE BULLET.
There maybe some correlation with twist rate and velocity of the bullet depending on the bullet composition.
Stabilization of the bullet depends on the weight, diameter, length and twist rate AND velocity can effect it.
NEXT:
Twist Rate x Velocity = RMP
You can have a high RPM that the bullet can fail depending on the bullet composition.
So in a nutshell your TWIST RATE stays the same no matter the length of the barrel and effects "Stability"
Length of the barrel can effect the "Velocity" but not really the "Stability"

If you are reloading and want to see if the bullets you are shooting will stabilize in your rifle-Get your twist rate then info on your bullet and input into a Bullet Stabilization Program.

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

 

[Paladin300]

Here's a question that occurred to me recently.

What effect does twist rate have on pressure? IE, If I shoot a 55 gr bullet out of Rem factory 22-250 with a 1-14" twist rate, and shoot the same round in a 1-8" twist rifle, am I going to run into excessive pressure earlier in the fast-twist round.

And related, how does accuracy generally compare? Any advantage of fast vs slow twist, assuming both twists stabilize the same bullet?

The answer to the question is yes. A faster twist does affect pressure but not as significant as you would think. Increased pressure equals increased velocity. I think the faster twist allows you to shoot the same velocity with a slightly lower charge. So there is not a significant difference in velocity. As far as accuracy goes I find that faster twist bullets are more accurate and precise. That said I also find that the accuracy node is usually at a lower charge weight. When you really push up the pressure on any barrel in my experience they become less accurate and precise. For example I can shoot 265 gr pills out of my 338 at well over 3000 fps however it really likes them at around 2850. Just my observations and thoughts.

 

[vedauvoo]

Thanks Cape Cove,

Interesting. So there would be no disadvantage in opting for a fast twist vs slow twist, as long as the bullet doesn't blow up?

 

[Paladin300]

Thanks Cape Cove,

Interesting. So there would be no disadvantage in opting for a fast twist vs slow twist, as long as the bullet doesn't blow up?

Very few modern bullets blow up from being over twisted. In my experience it comes from trying to push them to fast or hard and then we are mainly talking about light varmint bullets and light weight for caliber target bullets like Berger.

 

[cape cove]

Thanks Cape Cove,

Interesting. So there would be no disadvantage in opting for a fast twist vs slow twist, as long as the bullet doesn't blow up?

What I have been told is that the best twist you can have is the slowest twist that stabilizes the bullet that shoots best in your rifle. Kind of a "catch 22". The faster the twist the more magnified any flaw in your bullet has and this will show on the target. Again this is something that I have not tested but was told by good bench rest shooters.

 

[Mikecr]

Stability is a matter of bullet build and drag. A bullet that is built inherently out of balance will tip with drag. If a bullet's center of gravity is ahead of center of pressure, or if there is no drag present, the bullet would be stable without further means (like twist rate) to provide it.
But bullets are not balanced with nose first flight. And unless shooting in outer space, there is drag against your bullets. So there are overturning forces in play.

We counter the overturning forces with gyroscopic forces, provided by turning the bullet at a sufficient rate –to overcome the overturning. The sufficient rate is not based on time, but on bullet balance and drag. Since bullet build is fixed, the field variable is drag. The drag source is some distance of air at some density. This is what I refer to as DISPLACEMENT.

I you launch a bullet at 2,000fps, or 10,000fps, with an initial twist rate of 9:1, that bullet covers 9" of air density, per turn, at either speed. Put another way, you initially have one turn to overcome 9" of effective drag-born overturning moment. This IS effective, because varying displacement presents varying effective twist rate –once freed from your barrel.

Now you can jump on feet per second and declare 'there is your TIME', but stability requirements are not expressed in time. They're expressed in displacement per turn,, with good reason: it's true. It passes all tests. If you try to muddle this up with time, in the form of RPMs, you get failed tests. No bullet makers would declare stability requirements in RPMS.

So how do people form the RPM and higher velocity notions for stability? Well, there are higher gyroscopic forces with higher RPMs. But if you're getting higher RPMs with higher velocity, you're also overcoming more drag with that higher velocity. Drag goes up at a square of velocity. And it happens that all this nearly cancels to a wash right there. However, there are aerodynamic matters in play, and drag isn't quite squared, but adjusted with a coefficient per mach number, which is not purely velocity, but tied to air density attributes (back to effective displacement). With enough effort here, it could be suggested that more RPMs, produced through higher velocity, nudge stability up (a tiny amount). I'll concede to that, but it won't help to think this will help.

If you run the numbers as tests, you can watch stability go all over the map, and all with the same RPMs. Right?
It's 20deg colder out there today and your stability drops some –with the same RPMs. Seems a failed test for RPM thinking…
What changed is effective displacement.
As far as thinking that the more time a bullet is in contact with fixed rifling, the faster it will turn?
That goes back to RPM thinking and time. But longer barrel acceleration, speed, and turns per time, do not change twist rate.
The fixed rifling fixed that. Locked it all into displacement per turn.

 

[Paladin300]

Stability is a matter of bullet build and drag. A bullet that is built inherently out of balance will tip with drag. If a bullet's center of gravity is ahead of center of pressure, or if there is no drag present, the bullet would be stable without further means (like twist rate) to provide it.
But bullets are not balanced with nose first flight. And unless shooting in outer space, there is drag against your bullets. So there are overturning forces in play.

We counter the overturning forces with gyroscopic forces, provided by turning the bullet at a sufficient rate –to overcome the overturning. The sufficient rate is not based on time, but on bullet balance and drag. Since bullet build is fixed, the field variable is drag. The drag source is some distance of air at some density. This is what I refer to as DISPLACEMENT.

I you launch a bullet at 2,000fps, or 10,000fps, with an initial twist rate of 9:1, that bullet covers 9" of air density, per turn, at either speed. Put another way, you initially have one turn to overcome 9" of effective drag-born overturning moment. This IS effective, because varying displacement presents varying effective twist rate –once freed from your barrel. An idea that changing barrel length affects the twist rate is false.

Now you can jump on feet per second and declare 'there is your TIME', but stability requirements are not expressed in time. They're expressed in displacement per turn,, with good reason: it's true. It passes all tests. If you try to muddle this up with time, in the form of RPMs, you get failed tests. No bullet makers would declare stability requirements in RPMS.

So how do people form the RPM and higher velocity notions for stability? Well, there are higher gyroscopic forces with higher RPMs. But if you're getting higher RPMs with higher velocity, you're also overcoming more drag with that higher velocity. Drag goes up at a square of velocity. So it happens that all this nearly cancels to a wash right there. However, there are aerodynamic matters in play, so drag isn't quite squared, but adjusted with a coefficient per mach number, which is not purely velocity, but tied to air density attributes (back to effective displacement). With enough effort here, it could be suggested that more RPMs, produced through higher velocity, nudge stability up (a tiny amount). I'll concede to that, but it won't help to think this will help.

If you run the numbers, as tests, you can watch stability go all over the map, and all with the same RPMs. Right? It's 20deg colder out there today and your stability drops some –with the same RPMs. Seems a failed test for RPM thinking… What changed is effective displacement. And that passes all tests.

As far as thinking that the more time a bullet is in contact with fixed rifling, the faster it will turn?
You just haven't thought that through. Bless your heart

What he said! Now can you put them cookies down on the low shelf so everyone can reach them! That was a great explanation. It is what I was trying to explain but I don't think everyone gets it and I wasn't doing a very good job of it! It is fairly simple but then again not!

 

[cape cove]

If you run the numbers, as tests, you can watch stability go all over the map, and all with the same RPMs. Right? It's 20deg colder out there today and your stability drops some –with the same RPMs. Seems a failed test for RPM thinking… What changed is effective displacement. And that passes all tests.

Great explanation Mike . I guess it would be the same as when the altitude increases. Stability goes up but RPM's remain the same.

 

[1mechanic]

Yes and in his words ( not trying to put words in his mouth ) the displacement goes down when the air density decreases.

 

[Mikecr]

I you launch a bullet at 2,000fps, or 10,000fps, with an initial twist rate of 9:1, that bullet covers 9" of air density, per turn, at either speed. Put another way, you initially have one turn to overcome 9" of effective drag-born overturning moment. This IS effective, because varying displacement presents varying effective twist rate –once freed from your barrel.

Let me expand on this with something fun (external ballistics) to think about.
We initially have 9" displacement per turn here, and let's say that this is providing a gyroscopic stability (Sg) of 1.2.
That's at the muzzle, on bullet release, and it's marginal. The bullet rarely releases perfectly clean, and it takes a few feet for gyroscopic forces to bring the bullet fully point forward. That first few feet of the muzzle represents the biggest gyroscopic stability challenge.
If the bullet does not tumble immediately, then it is well on it's way, with Sg climbing from there, all the way to transonic.

The Sg climbs because the effective displacement per turn is changing.
It's getting tighter and tighter, as the bullet velocity slows faster than it's turn rate.
An example Sierra 111 DTAC, ICAO air density, 3000fps, 9:1 twist

Muzz Sg = 1.16, Vel 3000, Eff Twist 9.0 [240krpm]
50yd Sg = 1.22, Vel 2917, ETW 8.8 [239krpm]
100yd Sg = 1.27, Vel 2834, ETW 8.6 [237krpm]
200yd Sg = 1.38, Vel 2674, ETW 8.2 [234krpm]
278yd Sg = 1.50, Vel 2553, ETW 7.9 [232krpm] Fully Stable (sleep)
500yd Sg = 1.88, Vel 2226, ETW 7.1 [226krpm]
1Kyd Sg = 3.34, Vel 1582, ETW 5.3 [215krpm]

I threw RPMs in just to show that RPMs are going down while Sg is going up.
Let's also look at hypothetical muzzle velocity affect to this bullet's Sg from 9tw:

3,000fps Sg = 1.16
3,500fps Sg = 1.19
4,000fps Sg = 1.21
10,000fps Sg = 1.34
17,513fps Sg = 1.50 fully stable, and on it's way to Proxima Centauri at 1.4 million RPMs.

Ok, that's fun right there. I don't care who you are

 

[QuietTexan]

I don't see how it couldn't. The projectile is exposed/influenced by more rifling

If the rifling is well cut and constant, there's no additional "influence" once the turn has started. It really only takes the length of the bullet bearing surface to gain the full twist imparted by the rifling. After that the bullet rides down the tracks engraved in the jacket by the lands. If anything a longer barrel has more opportunities to have the lands be cut unevenly, and that would do more damage to a bullet jacket than a shorter barrel that has fewer inconsistencies.

Bullets blowing up can be caused by multiple things. Very long bullets can twist before the entire bearing surface is engraved into the rifling lands. Very thin jackets can have a higher percentage of metal displaced away from the core. If either of those happen the bond between the core and jacket can break, and the bullet gets torn apart by its own off-axis forces after it leaves the barrel. Bond can mean both chemical bonding used in bonded bullets, or simply the frictional/interference fit between core and jacket.

An exception to all this is gain twist barrels, in those the bullet has to pass through the the fastest point of the rifling to have that spin imparted on it. If you cut a gain twist down the final twist won't be what was specified, it'll be somewhere between the two specs. If you look at bullets that come out of gain twists the rifling marks are wider than the lands because of constantly re-engraving during the twist. If the rate is correct and the bullet isn't designed too lightly there's no negative side effects to the wider (but not deeper) tracks. Since the bullet is continually re-engraving itself onto the lands there is a small energy loss, but seeing as how a primer alone can get some bullets so engraved they get stuck, it's a meaningless difference in the overall energy budget of the cartridge.

 

[Pdyson]

Yes, it is MV not barrel length determines RPM. But barrel length directly affects MV. The twist rate and velocity together affect RPM of the bullet. Longer barrels can increase the RPM by increasing the muzzle velocity but not enough to overcome a twist rate which is too slow to begin with. It might work if you are marginal but if you are going to fit a new barrel, get one with the right twist rate.

 

[Mikecr]

Stretching to imply that barrel length has anything to do with stability is possibly how OP got started with his thinking.

Like when having 9tw, but needing 8tw, someone somewhere posts 'well add barrel length then'.
And then someone new to this and receiving that is thrown right into wrong thinking..
I suspect that the RPM nonsense began this way.

That's not what we want to do to anyone here.

 

[stazmania]

No matter if the bullet comes out of a 6" barrel or a 6' barrel if the bullets and velocity is the same, the bullets will have the exact same result, impact, rpm, drift and drop. velocity equals higher rpm but not rotation for distance. In my case/thinking if i am building a big magnum and not planning on shooting the heaviest highest bc. bullets, i will opt for a slightly slower twist. If I am building a gun of equal caliber but not so overbore/high velocity I will opt for a faster twist even with the same bullet weight, just be cuz it make me feel warm and fuzzy.