What happens when the rifling twist is too tight?

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C0untZer0

C0untZer0

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I am wondering what some of the symptoms of too-tight rifling twist are.

The reason I started thinking about this is someone suggested a pocket pistol chambered for the 5.7x28mm (Five SeveN), I think the problem would be too short a barrel length but I'm wondering how much that could be mitigated by using lighter bullets and a tighter rifle twist.
 
all i can say is shooting 223, 55fmj at around 3250 from a 1 in 7 that it shot 1 inch at 50 yards, and didnt make it on paper at 100. Puff of dust around 75 yards. These were cheap bullets, and only one brand did it, but seems they exploded. Hope that helps some.
 
If the rifling twist is too tight...

The bullet spins really, really fast.

MJSDWash is NOT the first person to have bullets spin themselves to death from overspin. P. O. Ackley wrote about it over sixty years ago shooting 40 grain bullets from a .219 Donaldson Wasp with an experimental 1-5 twist. They spun themselves so hard they came apart.

However, in general terms, a bunch of overspin is better than just a little underspin.

Reading your question about a short barreled FN 5.7; just what 'problem' do you foresee with such a short barrel? Accuracy or velocity performance?
 
just what 'problem' do you foresee with such a short barrel? Accuracy or velocity performance?
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That the 28gr - 31gr bullets would tumble / keyhole out of such a short barrel.

Pocket pistols aren't normally used as medium/long range pistols (although there was a debate about it here concerning sights on pocket pstols: http://www.thehighroad.org/showthread.php?t=717362&highlight=sights )

If a pocket pistol the size of a Kahr PM9 were chambered for the 5.7x28 I wouldn't be worried about not being able to hit targets out to 50 meters, I'd be more worried about terminal ballistics - inconsistent performance due to keyholing mainly..
 
The length of the barrel effects speed of the bullet and sight radius. Once the bullet is spinning fast enough for stabilization, its fine, for your purpose, a faster spin will not give you any advantage.
 
when the twist is to tight the bullet spins to fast and don't stabilize in flight. The bullet yaws and the nose of the bullet is tipped above the line of flight. when you get to the 200 yd mark you may very well get keyhole in the target and poor accuracy.
It is possible to spin them so fast that centrifical force causes them to explode in flight
 
Maybe I should ask the question a different way.

Could a 2.9" barrel sufficiently stabilize the 28gr-31gr 5.7x28mm bullets?
 
Another problem with a FMJ spinning too fast (even if not fast enough to break it) is actually over stabilization. When spun at it's orginal rate of 1:9 or even 1:14 a 55gr FMJ went pretty wonky when it hit something...the probably source of the old "M16 bullets tumble in flight" urban myth from a few decades ago. That same bullet spun up to 1:7 is still pretty darned stable when it hits and may tend to cause less damage at the target...with mileage varying greatly from case to case.

The debate quickly becomes, however, whether or not this really matters. Breaking apart? That would certainly matter!
 
That same bullet spun up to 1:7 is still pretty darned stable when it hits and may tend to cause less damage at the target...with mileage varying greatly from case to case.
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Decreased terminal performance caused by of 1:7 "over-stabilization" is a myth that has been debunked.

The reasons for inconsistent terminal performance have been identified as variations in angle-of-attack of individual projectiles and variations in fleet yaw of individual rifles, not "over-stabilization". More info on slide 7 of this presentation - http://www.dtic.mil/ndia/2008Intl/Roberts.pdf
 
Could a 2.9" barrel sufficiently stabilize the 28gr-31gr 5.7x28mm bullets?
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Lots of 40 grain .22's (5.56) running around out of short barrels. A 31 grain 5.7 is a short, fat bullet. Should be easy to stabilize, you can plug your numbers into the Greenhill Formula and see what it says for twist. Look up Greenhill Formula, or read about it on wikipedia about halfway down the rifling article.

http://en.wikipedia.org/wiki/Rifling
 
Shawn Dodson said:
Decreased terminal performance caused by of 1:7 "over-stabilization" is a myth that has been debunked.

The reasons for inconsistent terminal performance have been identified as variations in angle-of-attack of individual projectiles and variations in fleet yaw of individual rifles, not "over-stabilization". More info on slide 7 of this presentation - http://www.dtic.mil/ndia/2008Intl/Roberts.pdf
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Wow...that was interesting. Thanks.
 
Taurus tried that a few yrs back with a revolver called the instant back up. The thing was a flop because the bullets would push back out of the cylinder against the frame after a couple of shots rendering the pistol useless until you pulled the spent shells out.
 
centrifical
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centripetal

Gain twist rifling is used in some guns when a model is made with a shorter bbl than the fast cartridge was originally designed for.

My S&W 460 magnum with a 5" bbl is an example. The extreme speed of the cartridge was originally designed to be fired from an 8 and 3/8ths inch bbl. They put gain twist rifling in it to stabilize the bullet. And it works great.

Something similar could be done to improve the accuracy of a 5.7 with a short bbl. But for SD distances, is that necessary? Probably not.
 
Regarding a 5.7 'pocket pistol' - Keep in mind that the overall length of the factory cartridges is over 1.5" ... and that auto loader barels are usually measured from the breech face.

The actuall effective barrel length of a 5.7 pocket pistol would be very short. a 3" barrel would only give you 1.4" of rifling give or take. I think this would put a serious damper on performance.

Also, the grip would have to remain relatively long across the hand to accomidate the cartridge.

Also, you would get a tremendous fireball only equaled by the .223 AR pistols - bad for defence and low light shooting.
 
As a general rule, the lighter the bullet, the slower the twist required to properly stabilize the bullet in flight.
 
In my humble experience, nothing. The .223 Rem 45 gr bullets out of a 1:7 twist barrel went to the point of aim.
 
Overspin will ruin the accuracy of your gun. If the bullets are spinning to fast they will behave like a frisbee or a football with to much spin and rise instead of a predictable arc.
 
What's an in-flight smoker called. I forget. A blue streak? Under stabilized bullets have a tendency to tumble on impact. The longer the bullet is in FMJ configuration with a slow twist the more it tends to tumble. Rifling is not set up for bullet weight but bullet length which may or may not increase with bullet weight.

To answer the OP's question...I do not know but uncorking that round in a pocket blaster would have two people, assuming that Momma's choir boy survives, having to have written down what the lawyers are saying.:eek:
 
I will answer it from a different perspective. For home defense distances, the answer is you will be fine.
 
I am wondering what some of the symptoms of too-tight rifling twist are.
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It's not really something most people need to worry about. Generally the problems occur when people push the envelope in one way or another.

With that in mind, here are some of the issues that may be encountered.

1. http://www.bergerbullets.com/wp_super_faq/why-do-you-recommend-the-slowest-twist/ According to Berger bullets, spinning a bullet faster than is needed "can amplify any inconsistency in the bullet" which may result in some level of degradation in accuracy.

2. In extreme cases the bullet may be spun so fast that it can't hold together.

3. It's possible to create a situation where the bullet is "overstabilized". It's not usually an issue at short ranges but more of a problem as the range stretches. What happens is that the bullet is spinning so fast that it resists changing orientation to follow the arc of the trajectory. We know that the bullet follows an arc--it starts out aimed upwards a little bit, then at midrange it levels out and then it starts back down toward the ground as gravity pulls it downward.

A properly stabilized bullet will point in the direction of travel--there's enough spin to keep it generally oriented correctly (prevents tumbling) but not so much gyroscopic force that the air resistance can't keep the nose pointed in the direction the bullet is actually going. So the bullet points up when the bullet is going up on the first part of the trajectory, level when it's level briefly at midrange, and down when the bullet starts back down. The spin/gyroscopic force keeps the bullet from tumbling and the air resistance keeps the nose pointing in the direction of travel.

If the bullet is overstabilized there's so much gyroscopic force that it won't allow air resistance to re-orient the bullet as it travels its arc. That means that the bullet is now not as efficient a projectile as it should be since it eventually ends up being pointed a little bit away from the direction it's actually travelling. If it starts out pointing a little bit up, it stays pointing in that direction all the way to the target. Obviously during the level and downward parts of the arc, the improper orientation increases air resistance and decreases the ballistic coefficient of the bullet.
Could a 2.9" barrel sufficiently stabilize the 28gr-31gr 5.7x28mm bullets?
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The spin required for stabilization is a function of the length of the projectile, not the weight. We often talk about it in terms of weight since, for a given caliber, bullets of similar construction which weigh different amounts are generally different in length as well.

However, not all bullets are made the same way. A .224" 28gr bullet with a steel core will be longer than a .224" 28gr bullet with a lead core, for example.

What stabilizes a bullet is spin rate. That is, how fast the bullet is spinning. The spin rate of the bullet is dependent on the muzzle velocity and the rifling twist rate. For a given rifling twist, a higher muzzle velocity will result in a faster spin rate. For a given muzzle velocity, a faster twist will result in a faster spin rate.

So with a very short barrel, the muzzle velocity might be so low that the rifling twist rate that stabilizes a given bullet in a longer barrel (with a higher muzzle velocity) wouldn't be sufficient to stabilize that bullet in the short barrel.

It's not a difficult calculation once you have all the variables figured out. Fortunately other folks have worked the problem for us.

http://kwk.us/twist.html

You will need to know the muzzle velocity to work the problem.
 
It's the third case I discuss in my post. As far as being able to document it to provide specific examples, that would require careful comparison testing under otherwise identical conditions with different rifling twists and/or very sophisticated equipment.

The effect is, from a practical standpoint, a degradation of the effective ballistic coefficient of the projectile. In other words, by overstabilizing the projectile, you've now made it point in a direction that is not precisely the same direction it is traveling for most of its trajectory.

That will make it a poorer projectile than it should be. It's going to slow faster and drop more than it should based on the projected trajectory derived from the nominal ballistic coefficient. If you've paid extra for a premium performance bullet (e.g. a match boat-tail hollowpoint made with an exemplary ballistic coefficient to maximize long-range accuracy), you won't be getting what you paid for.

It's probably an overstatement to say that it will ruin the accuracy of the gun, but it's certainly not helping things.
 
It's been a while since I had to do this for grades or money, but I think it works like this...

ALL OTHER FACTORS BEING EQUAL (and they almost never are), faster twist causes higher pressure for the same projectile mass and acceleration. Faster twists tend to stabilize heavier (thus longer) projectiles better than lighter (thus shorter) projectiles.

If the goal is to launch a lighter projectile for a shorter barrel to the same velocity as a heavier projectile from a longer barrel, a more gradual twist rate in the shorter barrel enhances stabilization in flight, and reduces pressures.

Not being a fan of the the 5.7x28mm, and still believing that projectile cross sectional area and momentum are better predictors of incapacitation, I find the value of achieving a high, but not extremely high muzzle velocity (Mach 1 < muzzle velocity < Mach 2) with a lighter projectile elusive. Projectile kinetic energy at impact correlates poorly with incapacitation probability, unless impact velocity exceeds Mach 3 (at which point projectile blast wave amplitude is sufficiently large, and compressibility of aqueous media sufficiently small that individual water molecules behave as secondary projectiles, inducing cavitation and hemorrhage).
Multiple through & through thoracic penetration by projectiles w/2.5 times the cross-sectional area and 4 or 5 times the mass of those launched by the 5.7 were repeatedly ineffectual in terminating determined attacks (Jolo, Phillipines, @ 1906). Conversely, torso hits from projectiles having 7-8 times the mass, 2X the diameter, and @ 1/3 the velocity of 5.7x28mm projectiles, were reported 60% to 80% effective in terminating attacks, whether they completely traversed the attacker's torso, or not.
Despite the 5.7's catastrophic efficacy during the Ft. Hood atrocity, data concerning its effect in other situations is insufficient to support even preliminary conclusions about its general value as a fight-stopper. Attenuating its performance for the sake of portability would seem a step in the wrong direction.
YMMV
 
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