stabilization?

[brian923]

what is the main effects of velocity, trajectory, and accuracy on a bullet that is over stabilized?

i have a 1/8 twist 7 mm barrel, and am looking to re chamber. i would need to use heavier bullets due to the faster twist rate, but i want to know what effects it will have on the lighter bullets. thanks guys, Brian

 

[Shelley]

This is the way it was explained to me:

1. If the spin is really too fast the bullet will disintegrate.

2. If the bullet is just spinning faster than needed, any inconsistency in the bullet jacket and center of mass will cause the bullet to wobble a bit. The faster the spin the worse the wobble. With perfect bullets there would be, in theory, no down side to a faster than needed spin.

Shelley

 

[people]

You should be ok with shooting lighter bullets in 7mm.

 

[brian923]

only 1350 fps? thats pretty slow for a .223 isnt it? brian

 

[ConnorExum]

How do you overstablize something? That is impossible. It is either stable or not stable. The problem is clearly not overstablized but over accerlatered thus casing a different failure.

 

[jb77]

Connor,I disagree. It is not about being stable or unstable.It is about RPM's. Too many rpm's due to a twist rate that is too fast will accentuate any flaws in the bullet cores or the bullet jackets. In extreme case the jacket will fail and disintegrate. In most cases the instability will show up as yaw and accuracy will go out the window.

 

[Cheechako]

In most cases the instability will show up as yaw and accuracy will go out the window.

At normal velocities and twist rates, I think you would have a hard time proving that assumption. JMHO

Ray

 

[Asa_Yam]

How do you overstablize something? That is impossible. It is either stable or not stable.

It's not only possible, ballistics defines the conditions as follows:

- A projectile is unstable if the nose fails to point along the direction of flight following a disturbance.

- A projectile is stable in flight when the nose points along the direction of flight following a disturbance.

- A projectile is overstable in flight is if the nose fails to point along the direction of flight due to the spin of the projectile. Such a condition is normally encountered either in very high angle fire, or when the spin rate downrange is excessive. In either condition, the projectile retains its nose-up attitude despite the downward trajectory. If a bullet is fired into the air straight up, and returns to earth base first - it's overstabilized.

 

[Shelley]

As to Asa's explanation.... I'm well aware of the nose up at extreme range thing. But, would there be any down side to shooting lighter/shorter bullets at say 100 & 200 yards even if the spin is too much? Providing they hold together. The nose up thing shouldn't come into the equation till the bullet is further down range,I'd think).

I shoot a match each month that requires me to shoot six different yardages between 100 meters and 500 meters. I've been pondering the feasibility of shooting 100 grain flat base 6.5 bullets at short range because I need Benchrest accuracy at 100 meters where we shoot a score target and at 200 yards where we shoot a group target. At all of the longer ranges, just hitting the target scores. My thinking is that even in the 8 twist barrel perhaps the lighter and shorter bullet will score better, so far as precision goes, and the 123 grain VLDs will score better at the longer distances. What do you guys think?

I'll be testing the 100 grain bullets at 100 and 200 yards this Saturday to see if there is any advantage.

Shelley

 

[jb77]

Cheechako, i completely agree that at normal twist rates and velocity it is hard to over spin a bullet. But Shelley is talking about light bullets and using a fast twist rate. I have witnessed bullet blowup in several calibers under this scenario. I have never been able to achieve acceptable accuracy with light bullets and very fast twist rates.

Shelley,i would try to find one load combination that would work at all the ranges you are shooting at.

 

[people]

Darn my fast typing skills.

 

[gcabrera]

One possible downside of using fast twist "unnecessarily" is they tend -faster rifling- to generate more copper fouling. Accordingly they tend to give you a flyer more frequently than a slow twist barrel. You may get by with this flyer at long range shooting, possibly blaming it to a wind gust. At short range, they will surely destroy your aggregate.

Good shooting,

George

 

[ConnorExum]

I think it is a case of misusing the world stable. Aerodynamicist of which I know a few would disagree your defination because stable flight would mean a bullet that is correctly stablized and able to hit the target in question. Any other state would be something that is unstable.

 

[ConnorExum]

How do you overstablize something? That is impossible. It is either stable or not stable.

It's not only possible, ballistics defines the conditions as follows:

- A projectile is unstable if the nose fails to point along the direction of flight following a disturbance.

- A projectile is stable in flight when the nose points along the direction of flight following a disturbance.

- A projectile is overstable in flight is if the nose fails to point along the direction of flight due to the spin of the projectile. Such a condition is normally encountered either in very high angle fire, or when the spin rate downrange is excessive. In either condition, the projectile retains its nose-up attitude despite the downward trajectory. If a bullet is fired into the air straight up, and returns to earth base first - it's overstabilized.

Yeah I know what you're talking about but think about it... Once you reached a stable state you cannot over stablize it is a contradiction in terms. What you've even fact done is gone from the point of stable flight to another piont in the bullet's flight envelope that is unstable again. The fact that bullet is spinning excessively is not important because stable flight only occurs in a narrow portion of the bullet's flight envelop... That's at least how Aerodynamists would explain it's flight.

 

[Asa_Yam]

Yeah I know what you're talking about but think about it... Once you reached a stable state you cannot over stablize it is a contradiction in terms. What you've even fact done is gone from the point of stable flight to another piont in the bullet's flight envelope that is unstable again. The fact that bullet is spinning excessively is not important because stable flight only occurs in a narrow portion of the bullet's flight envelop... That's at least how Aerodynamists would explain it's flight.

See http://www.nennstiel-ruprecht.de/bullfly/stab.htm#header_stability
and
http://www.nennstiel-ruprecht.de/bullfly/fig15.htm

 

[ConnorExum]

Yeah I know what you're talking about but think about it... Once you reached a stable state you cannot over stablize it is a contradiction in terms. What you've even fact done is gone from the point of stable flight to another piont in the bullet's flight envelope that is unstable again. The fact that bullet is spinning excessively is not important because stable flight only occurs in a narrow portion of the bullet's flight envelop... That's at least how Aerodynamists would explain it's flight.

See http://www.nennstiel-ruprecht.de/bullfly/stab.htm#header_stability
and
http://www.nennstiel-ruprecht.de/bullfly/fig15.htm

Well the persons I spoke to in the ballistics field one being an aerodynamist laughed when I asked about overstablziation... He called it a myth. What he said was the bullet is no longer stable actually. But if you want to call it overstablized that is fine. I'm just telling you what I got in an email once when I emailed professor of Aerodynamics at Cal Tech about the question.

If you read the description of the bullet's flight it is not describing a dynamically stable bullet, that is a bullet that is stable completely throughout the bullet's flight envelope instead the bullet is only gryoscopically/statically stable meaning that bullet is yawing or pitching long longitudal axis, but in flight the bullet is not stable it flying in an unstable state where the bullet is unable to correct it's trajectory hence giving you stable flight. Which is how the professor described the flight envelop to me and corresponds well with my extensive research into model aircraft and rocket aerodynamics when I was a kid.

Think the term we should use to describe this flight is dynamicaly-stable. Therefore we'll avoid confusion with static and dynamic flight stablity issues.