I've always been under the (perhaps mistaken) impression that, ceteris paribus, a longer barrel will stabilize a given bullet "better than" (or more likely to stabilize) a shorter barrel, with a given twist rate. For example, take the Saiga .308s - 1 has a 16" bbl, and the other has a 21.5" bbl. Both have a 1 in 12ish/13ish twist (rather than a 1 in 10 like a lot of .308s). Assume for a moment that the 16" bbl will NOT stabilize (be accurate with) 174 gr bullets, let's say (which is probably true). Given that 'fact', is it or is it not the case that the 21.5" bbl COULD POSSIBLY BE / WOULD IN FACT BE more likely to stabilize that 174 grainer? Or is rifling twist rate is rifling twist rate is rifling twist rate - same results, regardless of barrel length? The basis of my homegrown theory is just that with more barrel length, the bullet has more time in the barrel to get up to, or close to, what I'm going to call "terminal rate" of bullet spin, whereas with a shorter barrel, the 'slippage' factor so to speak might prevent it from getting up to the 'terminal rate'. In other words, at what length of barrel does the bullet typically achieve the maximum 'terminal rate' of bullet spin - early on or later on in barrel length? See what I'm saying? This all from someone who has never even taken a single engineering or physics class. But I feel that I have a decent instinct for physics.
Basic physics question: Rifling twist rate affected by barrel length?
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The rate of spin is determined by the rate of twist and velocity. If you know the rate of twist and muzzle velocity, it does not matter how long the barrel was - the rate of spin is known.
Given a rate of spin, the success of stabilization depends on velocity (and air density). Bullet "stabilization" is a pretty complex precession caused by air-resistance and gyroscopic effects. If there is not enough velocity or not enough air, the bullet may fail to stabilize or become over-stabilized (fail to point where it is flying and keep pointing where the barrel was pointing while flying "sidewise").
Maybe that's what the people mean by saying the barrel length is important. Cut down the barel of a properly designed firearm, lose a few hundred fps and the bullets may not stabilize any more.
miko
Given a rate of spin, the success of stabilization depends on velocity (and air density). Bullet "stabilization" is a pretty complex precession caused by air-resistance and gyroscopic effects. If there is not enough velocity or not enough air, the bullet may fail to stabilize or become over-stabilized (fail to point where it is flying and keep pointing where the barrel was pointing while flying "sidewise").
Maybe that's what the people mean by saying the barrel length is important. Cut down the barel of a properly designed firearm, lose a few hundred fps and the bullets may not stabilize any more.
miko
I have heard of "progressive twist rate" barrels, but have no experience with them. The progressive twist barrel starts out at say 1 turn in 18 inches of forward bullet travel, then increases to say 1 turn in 12 inches of forward bullet travel near the muzzle.
Not sure if my numbers are correct, but that is what I have heard.
Not sure if my numbers are correct, but that is what I have heard.
pauli
Member
precisely.
benEzra
Moderator Emeritus
It's complex.
The short answer is, twist rate per se isn't affected by barrel length (i.e., the twist rate of a given barrel is independent of the barrel's length), but the twist rate required to stabilize a given bullet may vary by barrel length. That's because the barrel length helps determine the bullet's velocity, and a given rotational RPM is required to stabilize a given bullet at that velocity. However, the two variables are not independent, because higher velocity = more aerodynamic forces tending to upset the bullet, so as velocity increases, bullet RPM will also need to increase, but not necessarily linearly. So a bullet fired from a shorter barrel will need slightly less stabilization since it is going slower (less aerodynamic forces), BUT it will be spinning at a commensurately lower RPM (less gyroscopic forces) and the design of the particular bullet will determine how those two effects play out.
The short answer is, twist rate per se isn't affected by barrel length (i.e., the twist rate of a given barrel is independent of the barrel's length), but the twist rate required to stabilize a given bullet may vary by barrel length. That's because the barrel length helps determine the bullet's velocity, and a given rotational RPM is required to stabilize a given bullet at that velocity. However, the two variables are not independent, because higher velocity = more aerodynamic forces tending to upset the bullet, so as velocity increases, bullet RPM will also need to increase, but not necessarily linearly. So a bullet fired from a shorter barrel will need slightly less stabilization since it is going slower (less aerodynamic forces), BUT it will be spinning at a commensurately lower RPM (less gyroscopic forces) and the design of the particular bullet will determine how those two effects play out.
Vern Humphrey
Member
The increase in velocity produces a higher rate of spin -- but barrel length has no other effect.
Let's imagine a barrel with a 1 in 12 twist. The bullet makes one complete revolution for every foot it travels.
Now imagine we load the cartridge to produce a muzzle velocity of 2,000 fps. The bullet is spinning 2,000 revolutions per second when it leaves the muzzle (and continues to spin at that rate, of course.)
Now we up the charge to get a muzzle velocity of 3,000 fps -- and the bullet spins at 3,000 revolutions per second.
In the first case, a long bullet might not be stabilized at the lower rate, and upping the muzzle velocity (and hence the rate of spin) might well stabilize it.
Note that we didn't vary barrel length in this "thought experiment."
cracked butt
Member
Do a search on "greenhill formula"
Twist rate= (Constant)x((Bullet diameter^2)/bullet length))
The constant = velocity x .05
If you plug in the numbers, you will find that a slower moving bullet needs a faster twist rate to stabilize with all other things being equal.
Twist rate= (Constant)x((Bullet diameter^2)/bullet length))
The constant = velocity x .05
If you plug in the numbers, you will find that a slower moving bullet needs a faster twist rate to stabilize with all other things being equal.
scotty
Member
"If there is not enough velocity ot not enough air, the bullet may fail to stabilize or become over-stabilized (fail to point where it is flying and keep pointing where the barrel was pointing while flying "sidewise")."
I've had a problem wrapping my mind around the concept of over-stabilization until I read that sentence. :banghead: Then the lights came on.
Thank you, miko! That clears it up nicely.
I've had a problem wrapping my mind around the concept of over-stabilization until I read that sentence. :banghead: Then the lights came on.
Thank you, miko! That clears it up nicely.
MTMilitiaman
Member
Barrel length might have an affect on twist rate and bullet stability in that a particular bullet needs to acheive a specific spin rate, measured in rotations per minute or second, to maintain stable flight. This is determined directly by velocity which is largely affected by barrel length. Longer barrel length = higher velocity = higher rotational velocity = unless I am missing something, higher stability for longer bullets.
nextjoe
Member
Let me guess, you were an econ major?
Sleeping Dog
Member
This sounds like a sales gimmick and expensive machining. In a sense, all rifles are progressive twist rate, because the bullet speed is progressive. In the first few inches past the chamber, the bullet is moving slow, so it's twisting slow. RPM and FPS both increase as the burning propellant causes the bullet to accelerate.
Regards.
JohnBT
Member
The Italians were worried about preserving the secret of the progressive twist barrel at least as early as 1890. JT
"23.9.1890: Since none of the submitted models so far has satisfied the commission, the four Italian state factories/ government arsenals (of Brescia, Terni, Torino, Torre Annunziata) are formally charged with studying and presenting additional rifle models.
Mauser and Mannlicher, still in business, are given constant-twist barrels (as to keep the secret of the progressive twist to Italy) and a newly-developed 6,5 mm rimmed cartridge upon which they have to build their rifles."
"23.9.1890: Since none of the submitted models so far has satisfied the commission, the four Italian state factories/ government arsenals (of Brescia, Terni, Torino, Torre Annunziata) are formally charged with studying and presenting additional rifle models.
Mauser and Mannlicher, still in business, are given constant-twist barrels (as to keep the secret of the progressive twist to Italy) and a newly-developed 6,5 mm rimmed cartridge upon which they have to build their rifles."
Sleeping Dog
Member
Ok, there's sort of a testimonial to the gimmicky technology.
Mauser and Mannlicher somehow survived past 1890, even with their limited constant-twist barrels.
The Italian gun-makers must have thought they were on to some revolutionary improvement
They were wrong. Plus, maybe progressive twist made a little sense if all you have is fast-burning black powder. Dunno.
Regards.
Father Knows Best
Member
I'm no expert, but I seem to recall reading an article recently that mentioned a modern firearm with gain twist (aka "Progressive twist") rifling. I think it might have been the S&W X-frame revolvers (.500 Magnum, .460 XVR, etc.). The reason given was that the gain twist did two things:
1. it helped prevent the bullet from "stripping" upon engaging the rifling; and
2. as a bullet engages rifling and begins to rotate in one direction, the firearm wants to rotate in the opposite direction (one of Newton's laws at work here -- for every action there is an equal and opposite reaction). This force is felt as a rotational torque on the firearm. Gain twist rifling slows the rate of acceleration of the spin, which reduces the counter-rotational torque felt by the shooter.
Assuming that's all true (and I have no reason to doubt it), then I would bet that the effects are only significant when you have relatively lightweight firearms and extremely high velocities, e.g., a handgun firing bullets at 2,000+ fps.
1. it helped prevent the bullet from "stripping" upon engaging the rifling; and
2. as a bullet engages rifling and begins to rotate in one direction, the firearm wants to rotate in the opposite direction (one of Newton's laws at work here -- for every action there is an equal and opposite reaction). This force is felt as a rotational torque on the firearm. Gain twist rifling slows the rate of acceleration of the spin, which reduces the counter-rotational torque felt by the shooter.
Assuming that's all true (and I have no reason to doubt it), then I would bet that the effects are only significant when you have relatively lightweight firearms and extremely high velocities, e.g., a handgun firing bullets at 2,000+ fps.
Vern Humphrey
Member
That phenomenon is called "trail." It rarely happens at any practical rate of twist, and is usually seen when shooting almost straight up.
During the blitz in London, small caliber anti-aircraft bullets were sometimes found sticking point-up in tarred roofs.
atomchaser
Member
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Wouldn't the AA bullets sticking noise up in the roof tar have more to do with aerodynamic forces as the stopped their forward travel and fell back to earth?
Vern Humphrey
Member
Hatcher did experiements as I recall, that established that above a certain angle of launch, .30-06 bullets would return to earth base-first, and below a certain angle, nose-first. In between they became unstable and tumbled.
The point is, trail is unusual in small arms projectiles, and then only under certain circumstances -- such as very high angles of launch.
JohnBT
Member
Now that you mention it, I do remember seeing a S&W ad for the .460 revolver touting the gain twist barrel. John
From the Lilja site www.riflebarrels.com/articles/barrel_making/details_of_accuracy.htm
"Lilja considers a constant rate of rifling twist throughout the bore extremely important to accuracy. If the twist decreases, a bullet traveling down the barrel is not fully supported and has a chance to yaw, or wobble, while it's still in the barrel. Yaw is when the nose or base of a bullet spins around its center.
When Lilja started making barrels in 1985 he experimented with barrels with a twist that had a slight gain over the length of the bore. Lilja felt the gain in twist kept a more consistent grip on the bullet and kept it from wiggling in the bore. "To some extent my barrels were unique with the gain twist. And at the time I felt the slight gain in twist was beneficial to accuracy," he said.
A .22 centerfire Lilja barrel with a 1 in 14-inch gain twist was measured for uniformity of twist by an electro-optic measuring device of Photronic Systems Engineering Company of Bonsall, California. A graph of the barrel's twist showed six percent less than a 1 in 14-inch twist in the first inch of barrel ahead of the throat. The rifling increased to 1 in 14-inches ten inches down the bore. From there to the muzzle, 20 inches down the bore, the twist gradually increased to four percent faster than 1 in 14-inches.
However Lilja has quit using a gain twist. After years of comparing the accuracy of barrels with a gain twist and barrels with a standard twist, he has decided the gain twist offers no accuracy advantages. "You get just as good or better accuracy with a standard twist," he said, "just as long as the twist remains exactly the same the entire length of the bore.""
From the Lilja site www.riflebarrels.com/articles/barrel_making/details_of_accuracy.htm
"Lilja considers a constant rate of rifling twist throughout the bore extremely important to accuracy. If the twist decreases, a bullet traveling down the barrel is not fully supported and has a chance to yaw, or wobble, while it's still in the barrel. Yaw is when the nose or base of a bullet spins around its center.
When Lilja started making barrels in 1985 he experimented with barrels with a twist that had a slight gain over the length of the bore. Lilja felt the gain in twist kept a more consistent grip on the bullet and kept it from wiggling in the bore. "To some extent my barrels were unique with the gain twist. And at the time I felt the slight gain in twist was beneficial to accuracy," he said.
A .22 centerfire Lilja barrel with a 1 in 14-inch gain twist was measured for uniformity of twist by an electro-optic measuring device of Photronic Systems Engineering Company of Bonsall, California. A graph of the barrel's twist showed six percent less than a 1 in 14-inch twist in the first inch of barrel ahead of the throat. The rifling increased to 1 in 14-inches ten inches down the bore. From there to the muzzle, 20 inches down the bore, the twist gradually increased to four percent faster than 1 in 14-inches.
However Lilja has quit using a gain twist. After years of comparing the accuracy of barrels with a gain twist and barrels with a standard twist, he has decided the gain twist offers no accuracy advantages. "You get just as good or better accuracy with a standard twist," he said, "just as long as the twist remains exactly the same the entire length of the bore.""
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