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  #1  
Old 12-17-2009, 10:11 PM
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Default Is this true?

Smart people help me.


Quote:
Originally Posted by aplinker View Post
Longer barrels do put more spin because they have higher velocity. It's rotational velocity (spin per unit time) that matters for stabilization. That calculation program simply uses velocity, which is a function of barrel length. You wouldn't have the same linear velocity from the same loading if the barrel length were different.

To exaggerate the point... Let's say you have 2 barrels, one is 1/10 twist and short, the other is 1/20 and long. If the round coming out of the long barrel is at 2X the linear velocity of the short, then the rate of spin of the two barrels would be identical.
This was in response to a commit I made in the centerfire forum. http://www.calguns.net/calgunforum/s...d.php?t=250433

How can a barrel have a higher velocity?

My thoughts are... A 6" and 30" 1/7 barrel will both have a twist rate of 1/7. Velocity can change with many factors and will not increase indefinitely with barrel length. Each barrel would stabilize a x length bullet at x velocity. Yes you might get a slight velocity advantage by using a longer barrel and be able to stabilize a slightly longer bullet but it is not spinning faster.
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Last edited by 264charlie; 12-17-2009 at 10:31 PM..
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Old 12-17-2009, 10:14 PM
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One more thought... Regardless of velocity, If bullet exits a x twist barrel is it not spinning at x per inch?
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Old 12-17-2009, 10:17 PM
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I see the point hes making but im with you and i never would have thought it would really work like that. Should be interesting to see what the "smart people" have to say.
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Old 12-17-2009, 10:36 PM
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Agreed

I just don't see how an increase in velocity can increase the rate of spin. That said, I am sure someone knows why a faster bullet requires less twist to stabilize.
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Old 12-17-2009, 11:00 PM
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longer barrel will yield more stability but not greater spinning rate.
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Old 12-18-2009, 5:15 AM
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The RPS's (Revolutions per Second) of the bullet will change with the velocity.

Example:
1-10 twist (1 rotation per 10 inches) @ 1000fps (12000 inches per sec)

12000 / 10 = 1200 Revolutions per Second


1-10 twist @ 2000fps (24000 inches per sec)

24000 / 10 = 2400 RPS

The higher RPS at 2000fps will stabilize a longer bullet. That's why long bullets will stabilize in marginal barrel twists IF you're able to drive them fast enough.
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Old 12-18-2009, 7:01 AM
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More linear velocity=more rotational velocity with a given twist.
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Old 12-18-2009, 7:23 AM
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What rksimple said. That's just simple physics.
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  #9  
Old 12-18-2009, 7:24 AM
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Aplinker is correct. A practical example is that a 1/9 twist 16" long barrel won't stabilize 75 amaxs, but a 20" 1/9 twist barrel will. The additional velocity at the same twist rate will generate a higher rpm.
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Old 12-18-2009, 7:37 AM
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Barrel length doesn't play into the equation and that's where he's confusing you. He's essentially using barrel length synonomously with velocity and that's incorrect.

All you need to know are the twist and velocity.

The more velocity you have, the more rotations you get per unit of time since you're able to cover a greater distance.

Simple.
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Old 12-18-2009, 7:47 AM
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Thanks guys... I was thinking in terms of rotations per inch not RPMs.
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Old 12-18-2009, 7:49 AM
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RPM and barrel twist rate are completely independent of each other, though the ultimate bullet rpm is effected by velocity and more barrel length gives more velocity.
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Old 12-18-2009, 7:59 AM
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Quote:
Originally Posted by ar15barrels View Post
more barrel length gives more velocity
There are limits to that statement..
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Old 12-18-2009, 8:32 AM
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So theoretically couldn't a 1 in 10 .308 barrel at let's say 29in almost be on the verge of spinning to fast vs the same configuration in a 20in would be just fine?
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Old 12-18-2009, 8:33 AM
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Obviously providing you were getting full powder burn down all 29"?
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Old 12-18-2009, 8:49 AM
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Regardless of the velocity of a bullet it's spinning at the rate (x per inch) of a barrel at exit. Velocity + Rate = RPM. A barrel MAY allow hight velocity but not always.

I still believe this statement to be incorrect "Longer barrels do put more spin because they have higher velocity". A barrel in itself cannot always generate more velocity. I understand a bullet exiting a barrel at a greater velocity will require less rate of twist to stabilize.

Thank you all for the clarification.
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Last edited by 264charlie; 12-18-2009 at 8:58 AM..
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Old 12-18-2009, 8:58 AM
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Incorrect informationis way I don't like togo outside of this sub-forum. Thank you to all the "real life" experts that take your time to pass on knowledge and proven facts.
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Old 12-18-2009, 9:06 AM
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Quote:
Originally Posted by wildcard View Post
There are limits to that statement..
There are theoretical limits but not practical limits.
You can't (practically) purchase a barrel that's long enough to (theoretically) slow the bullet down.
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Old 12-18-2009, 9:08 AM
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Quote:
Originally Posted by DirtRacer151 View Post
So theoretically couldn't a 1 in 10 .308 barrel at let's say 29in almost be on the verge of spinning to fast vs the same configuration in a 20in would be just fine?
There is no "verge of spinning too fast" unless you are talking about varmint bullets that just turn into gray clouds about 15ft out of the muzzle.
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Old 12-18-2009, 9:10 AM
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Quote:
Originally Posted by DirtRacer151 View Post
Obviously providing you were getting full powder burn down all 29"?
It's not the powder burn that pushes the bullet.
It's the pressure that the burnt powder creates that pushes the bullet.
The powder can finish burning in 16" of barrel and the may still be lots of residual pressure at 29".
As long as there is enough residual pressure to offset the bullet drag within the bore, the bullet will continue to accelerate.
Once the residual pressure drops below the bore friction threshold, acceleration goes negative and the bullet starts shedding velocity.
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Old 12-18-2009, 9:13 AM
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Quote:
Originally Posted by usdmgtr View Post
Regardless of the velocity of a bullet it's spinning at the rate (x per inch) of a barrel at exit.
The spin rate (x per inch) is NOT a factor in bullet stability.
The rotational velocity is what matters.

As a bullet flies, it loses forward velocity MUCH faster than it loses rotational velocity.

Think about that for a bit.
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Old 12-18-2009, 9:16 AM
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Quote:
Originally Posted by usdmgtr View Post
I still believe this statement to be incorrect "Longer barrels do put more spin because they have higher velocity".
Perhaps it would make more sense to you if I re-phrased the above statement to "Longer barrels do put more rotational velocity because they have higher velocity".

That statement would be correct.
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Old 12-18-2009, 9:19 AM
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Old 12-18-2009, 9:35 AM
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Bullets actually become MORE stable as they fly.
The Litz book goes into great detail about this.
There are two things happening.
First, there is the most drag at the highest velocity and it takes more rotational velocity to remain stable when there is more drag.
Since there is so much drag, the forward velocity loss is greatest at the highest velocity and the bullet slows down quickly.
The rotational drag is not much different though so the bullet does not lose nearly as much rotational velocity as it flies.

Second, as the bullet loses forward velocity, it takes less rotational velocity to stabilize it, yet the bullet has not lost nearly as much rotational velocity as it has lost forward velocity.
Therefore, the bullet has much more rotational velocity than it really needs, giving much better gyroscopic stability.

If Jason reads into this carefully, it will explain how bullets "go to sleep".
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Old 12-18-2009, 9:43 AM
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Quote:
Originally Posted by DirtRacer151 View Post
So theoretically couldn't a 1 in 10 .308 barrel at let's say 29in almost be on the verge of spinning to fast vs the same configuration in a 20in would be just fine?
Depends on the bullet and powder used. A real world example was John Whidden's Palma rifle that had a 32" 1:10. He used Berger's 155 vld to win the Palma team tryouts in 2004.

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Old 12-18-2009, 9:45 AM
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Quote:
Originally Posted by ar15barrels View Post
Bullets actually become MORE stable as they fly.
The Litz book goes into great detail about this.
There are two things happening.
First, there is the most drag at the highest velocity and it takes more rotational velocity to remain stable when there is more drag.
Since there is so much drag, the forward velocity loss is greatest at the highest velocity and the bullet slows down quickly.
The rotational drag is not much different though so the bullet does not lose nearly as much rotational velocity as it flies.

Second, as the bullet loses forward velocity, it takes less rotational velocity to stabilize it, yet the bullet has not lost nearly as much rotational velocity as it has lost forward velocity.
Therefore, the bullet has much more rotational velocity than it really needs, giving much better gyroscopic stability.
I'm guessing this is from chapter 10. I'm saving the heavy stuff since I'm at the in-laws all next week.
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Old 12-18-2009, 10:02 AM
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Quote:
Originally Posted by ar15barrels View Post
If Jason reads into this carefully, it will explain how bullets "go to sleep".
Already cut down my max 89gr load to 86 and 87. Testing it out this weekend to see if I can get it to stabilize closer to the barrel

Something you can ignore it all and the stars and planets all seem to coincidentally line up. And sometimes you have to experience it for yourself amd listen to the science/data. I'm currently doing the latter.
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Old 12-18-2009, 10:26 AM
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Quote:
Originally Posted by ar15barrels View Post
Bullets actually become MORE stable as they fly.
The Litz book goes into great detail about this.
There are two things happening.
First, there is the most drag at the highest velocity and it takes more rotational velocity to remain stable when there is more drag.
Since there is so much drag, the forward velocity loss is greatest at the highest velocity and the bullet slows down quickly.
The rotational drag is not much different though so the bullet does not lose nearly as much rotational velocity as it flies.

Second, as the bullet loses forward velocity, it takes less rotational velocity to stabilize it, yet the bullet has not lost nearly as much rotational velocity as it has lost forward velocity.
Therefore, the bullet has much more rotational velocity than it really needs, giving much better gyroscopic stability.

If Jason reads into this carefully, it will explain how bullets "go to sleep".
Another additional tidbit. At LONG RANGE it is theoretically possible to "over stabilize" the bullet. The bullet's trajectory is a asymmetrical arc; at the top of the arc the bullet should nose over so the longitudinal centerline of the bullet tracks with the bullet trajectory. This is maintain the effective BC of the bullet throughout the bullet's trajectory.

If the bullet is over stabilized, it can fail to properly nose over and adopts a nose high attitude. At this attitude the longitudinal centerline is not tracking with the bullet trajectory. As a result the BC of the bullet is degraded and may not track properly.

This optimal stabilizing is call "Balanced Flight" and is usually achieved by barrel twist rate AND micro knurling on the bullet (to increase rotational drag).

Over stabilizing is generally not a practical issue that becomes observable except for precision shooting at LONG Range.
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Old 12-18-2009, 11:38 AM
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I get it now

A bullet will maintain angular momentum as it decelerates and will increase past its original rate of twist (in distance). Twist rate is used with velocity to get a bullet to a specific rotational velocity required to stabilize in flight.
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Old 12-18-2009, 1:31 PM
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Quote:
Originally Posted by usdmgtr View Post
I get it now

A bullet will maintain angular momentum as it decelerates and will increase past its original rate of twist (in distance). Twist rate is used with velocity to get a bullet to a specific rotational velocity required to stabilize in flight.
The bullet actually noses over and "traces" with the trajectory.
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Old 12-18-2009, 1:59 PM
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Lost River Ballistic Technologies/Chey-Tac are the ones that fleshed out and patented the "Balanced Flight" concept. Theoretically, in balanced flight, the bullet never becomes unstable as long as there is forward velocity. The bullet transitions from supersonic to transonic to subsonic, maintaining it's trajectory. It does not experience buffeting or shockwaves. At that point the major obstacle to hitting a live target is time of flight (will the target remain stationary long enough for the bullet to arrive). For stationary targets, the determining factor would be minimum energy required on target will be the maximum engagement range.

Lost River/Chey-Tac use a patented design lathe turned Cu alloy bullet and also a proprietary rifling configuration to achieve this "Balanced Flight".
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Old 01-05-2010, 9:18 AM
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buffybuster,

I respectfully disagree with your description of the effects of bullets being 'over-stabilized'. It's a commonly held belief that bullets spinning 'too fast' will fail to trace as well as bullets that are 'optimally' stabilized. In fact, faster spinning bullets will not have a problem keeping their noses aligned with the velocity vector in the vertical plane, but faster spinning bullets will have an increased 'yaw of repose', which is when the nose points to the side, causing spin drift. However the yaw of repose is so small as not to affect the BC by even 0.1%.

This report was written to investigate the claimed BC of a particular .338 caliber bullet. The BC derived from drop data suggested an extraordinarily high BC. It was theorized that the bullet may be flying with a nose-up orientation as you describe, which was generating lift for the bullet causing it to hit higher even though it was flying with an effectively reduced BC (from induced drag).
The part of the report that's germane to this discussion starts around page 8 where 6-DOF simulations are performed for this bullet at various twist rates and the effects on vertical and horizontal POI at long range are examined.

Balanced flight, as you describe, is not a special case. It's just what naturally happens for spin stabilized bullets at supersonic speeds. Having a bullet remain stable thru the transonic flight speed is a different, bigger challenge, and can be improved by smart projectile/barrel design. If this is what Chey Tac/Lost River has achieved by design then it's significant. But if your claim is that only their 'Balanced flight' projectiles can actually trace in the supersonic part of the trajectory when others don't, then I disagree.

I've been reading on this forum for a while and this is my first post. I regret that it may appear argumentative, but I'm compelled to comment when I come across things like this. I hope no offense is taken, as none is intended.

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Old 01-05-2010, 9:28 AM
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Hey Brian, welcome....
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Old 01-05-2010, 10:53 AM
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I've been reading on this forum for a while and this is my first post.
Bryan,

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Old 01-05-2010, 10:55 AM
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buffybuster,

I respectfully disagree with your description of the effects of bullets being 'over-stabilized'. It's a commonly held belief that bullets spinning 'too fast' will fail to trace as well as bullets that are 'optimally' stabilized. In fact, faster spinning bullets will not have a problem keeping their noses aligned with the velocity vector in the vertical plane, but faster spinning bullets will have an increased 'yaw of repose', which is when the nose points to the side, causing spin drift. However the yaw of repose is so small as not to affect the BC by even 0.1%.

This report was written to investigate the claimed BC of a particular .338 caliber bullet. The BC derived from drop data suggested an extraordinarily high BC. It was theorized that the bullet may be flying with a nose-up orientation as you describe, which was generating lift for the bullet causing it to hit higher even though it was flying with an effectively reduced BC (from induced drag).
The part of the report that's germane to this discussion starts around page 8 where 6-DOF simulations are performed for this bullet at various twist rates and the effects on vertical and horizontal POI at long range are examined.

Balanced flight, as you describe, is not a special case. It's just what naturally happens for spin stabilized bullets at supersonic speeds. Having a bullet remain stable thru the transonic flight speed is a different, bigger challenge, and can be improved by smart projectile/barrel design. If this is what Chey Tac/Lost River has achieved by design then it's significant. But if your claim is that only their 'Balanced flight' projectiles can actually trace in the supersonic part of the trajectory when others don't, then I disagree.

I've been reading on this forum for a while and this is my first post. I regret that it may appear argumentative, but I'm compelled to comment when I come across things like this. I hope no offense is taken, as none is intended.

-Bryan
Bryan,

Welcome to the forum. Good discussion and substantive information is ALWAYS welcome. While reading your HATS report, had a similar hypothesis. I did not and do not claim that Chey Tac/Lost River are the only ones with 'Balanced Flight'. Though they do seem to be the only ones currently actively promoting it. Chey Tac's claim is their propriety combination of projectile and rifling profile will remain stable throughout supersonic, transonic, and subsonic transitions and maintain it's calculated trajectory (never deviates from trace).

Have/Can you run the 6-DOF simulation out to 2500yds. I'd be curious what happens to the bullet's angle of repose at those extreme ranges.

Trying to learn from the source.

Henry
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  #36  
Old 01-05-2010, 11:02 PM
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Originally Posted by BryanLitz View Post
buffybuster,

I respectfully disagree with your description of the effects of bullets being 'over-stabilized'. It's a commonly held belief that bullets spinning 'too fast' will fail to trace as well as bullets that are 'optimally' stabilized. In fact, faster spinning bullets will not have a problem keeping their noses aligned with the velocity vector in the vertical plane, but faster spinning bullets will have an increased 'yaw of repose', which is when the nose points to the side, causing spin drift. However the yaw of repose is so small as not to affect the BC by even 0.1%.

This report was written to investigate the claimed BC of a particular .338 caliber bullet. The BC derived from drop data suggested an extraordinarily high BC. It was theorized that the bullet may be flying with a nose-up orientation as you describe, which was generating lift for the bullet causing it to hit higher even though it was flying with an effectively reduced BC (from induced drag).
The part of the report that's germane to this discussion starts around page 8 where 6-DOF simulations are performed for this bullet at various twist rates and the effects on vertical and horizontal POI at long range are examined.

Balanced flight, as you describe, is not a special case. It's just what naturally happens for spin stabilized bullets at supersonic speeds. Having a bullet remain stable thru the transonic flight speed is a different, bigger challenge, and can be improved by smart projectile/barrel design. If this is what Chey Tac/Lost River has achieved by design then it's significant. But if your claim is that only their 'Balanced flight' projectiles can actually trace in the supersonic part of the trajectory when others don't, then I disagree.

I've been reading on this forum for a while and this is my first post. I regret that it may appear argumentative, but I'm compelled to comment when I come across things like this. I hope no offense is taken, as none is intended.

-Bryan
Thank you for posting...
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Old 01-06-2010, 9:20 PM
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Quote:
Originally Posted by BryanLitz View Post
buffybuster,

I respectfully disagree with your description of the effects of bullets being 'over-stabilized'. It's a commonly held belief that bullets spinning 'too fast' will fail to trace as well as bullets that are 'optimally' stabilized. In fact, faster spinning bullets will not have a problem keeping their noses aligned with the velocity vector in the vertical plane, but faster spinning bullets will have an increased 'yaw of repose', which is when the nose points to the side, causing spin drift. However the yaw of repose is so small as not to affect the BC by even 0.1%.

This report was written to investigate the claimed BC of a particular .338 caliber bullet. The BC derived from drop data suggested an extraordinarily high BC. It was theorized that the bullet may be flying with a nose-up orientation as you describe, which was generating lift for the bullet causing it to hit higher even though it was flying with an effectively reduced BC (from induced drag).
The part of the report that's germane to this discussion starts around page 8 where 6-DOF simulations are performed for this bullet at various twist rates and the effects on vertical and horizontal POI at long range are examined.

Balanced flight, as you describe, is not a special case. It's just what naturally happens for spin stabilized bullets at supersonic speeds. Having a bullet remain stable thru the transonic flight speed is a different, bigger challenge, and can be improved by smart projectile/barrel design. If this is what Chey Tac/Lost River has achieved by design then it's significant. But if your claim is that only their 'Balanced flight' projectiles can actually trace in the supersonic part of the trajectory when others don't, then I disagree.

I've been reading on this forum for a while and this is my first post. I regret that it may appear argumentative, but I'm compelled to comment when I come across things like this. I hope no offense is taken, as none is intended.

-Bryan
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  #38  
Old 01-06-2010, 9:22 PM
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that didn't work the way I wanted it to....

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I must go...I must smell that smell...The smell of burnt gunpowder blowing down the line, the sweet sound of lead connecting with metal...I have to have it and hear it....I know you know what I mean, I know Bob would know what I mean.... Its nothing that a little steri strips and some tegaderm won't fix...

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  #39  
Old 01-06-2010, 9:55 PM
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That Brian Litz guy IS a rocket scientist.

From the middle of this page:

Quote:
About the Author

Bryan Litz earned his Aerospace Engineering degree from the Pennsylvania State University in 2002. For the next 6 years, he worked for the US Air Force on air-to-air missile design, modeling and simulation. In 2008, Bryan became the Chief Ballistician for Berger bullets.
Bryan is also a very active and successful shooter. Some significant accomplishments include: US National Palma Champion (2008), firing member of the winning 2008 US Team in the Spirit of America International Rifle Match, and National Record Holder for the NRA iron sight midrange course (450-39X).
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  #40  
Old 01-07-2010, 9:02 AM
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that was my point...he is the rocket scientist, not me....
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