Twist rate and velocity

[nickorette]

Are they related? Or is the performance only effected by bullet weight?

Say you had a 1 in 7 twist barrel in .223. 70 grainers shot perfect out of it at a velocity of 3000 fps. If you loaded up a 55 grainer at 3000 fps would you get the same resuts? (Assuming BC/S.D. and all other conditions were the same).

 

[H4831]

Are they related? Or is the performance only effected by bullet weight?

Say you had a 1 in 7 twist barrel in .223. 70 grainers shot perfect out of it at a velocity of 3000 fps. If you loaded up a 55 grainer at 3000 fps would you get the same resuts? (Assuming BC/S.D. and all other conditions were the same).

Would you run that by me again, please!














1

 

[ben hunchak]

Not sure what yer gettin' at here...but in my experiments, when bullet length vs twist is at outer limits, the velocity can be critical. An example is using 180gr. bullets in a 1-13" twist which I am experimenting with in a .300 Whisper, at subsonic speeds, bullets are showing very slight yaw. Groups were poor and vertical stringing was very evident, upping the speed to 1650fps, and all the problems went away, groups were under 1/2" and showed no vertical tendency.
Don't get me wrong, these results didn't happen overnight, as cast bullets can take alot of experimenting and alot of load development, but the end result shows how critical velocity can be in some cases.

 

[fat but funky]

I think I know what you are asking.

To paraphrase - What is the relationship between twist rate, velocity, and bullet weight when it comes to accuracy?

I will give you my understanding - though more experienced folks may have something else (possibly contradictory) to say.

point 1.) If we hold muzzle velocity constant, then longer bullets tend to need faster twists than shorter bullets. It is RPM that stabilizes the bullet. So, with muzzle velocity held constant, the only way to regulate RPM is with twist rate.

point 2.) if we hold bullet construction constant, then longer bullets will be heavier than short bullets. Therefore, if we incorporate point 1 - heavier bullets need faster twist rates.

point 3.) Maximum muzzle velocity is regulated by the amount of powder that can be usefully burned. So things like allowable pressure, overbore status, bearing surface of the bullet, bullet weight, etc will all determine that amount. Heavier bullets, through their mass and generally increased bearing surface will not leave the barrel with as much speed as lighter bullets.

If you play with all three points, you see that in general, light (therefore short) bullets have a wide range of muzzle velocity where they tend to be accurate. Heavy (long) bullets need to achieve a certain RPM to be stabilized so they need a faster twist and/or loads that tend to be close to maximum to achieve that RPM.

Of course, some will say that it is possible to overspin a bullet such that it will either disintegrate or won't "nose down" at long range - this is true but doesn't really speak to your question. Also realize that bullet construction is anything but constant.

Fat

 

[Obtunded]

Mathematician George Greenhill was able to mathematically establish that bullet stabilization was influenced by its length in relation to its twist rate.

In other words, it does not matter what a bullet weighs, but how LONG a bullet is in relation to its width which determines optimal twist.

the Greenhill Formula:

Twist = 150 X D2/L

Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

Is a highly effective and predictable means of determining optimal twist rates. Having said this, there was a difference when bullets traveled closer to a trans-sonic speed. the factor if 150 must be changed to 180 in near trans-sonic bullet velocities.

 

[juanvaldez]

Mathematician George Greenhill was able to mathematically establish that bullet stabilization was influenced by its length in relation to its twist rate.

In other words, it does not matter what a bullet weighs, but how LONG a bullet is in relation to its width which determines optimal twist.

the Greenhill Formula:

Twist = 150 X D2/L

Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

Is a highly effective and predictable means of determining optimal twist rates. Having said this, there was a difference when bullets traveled closer to a trans-sonic speed. the factor if 150 must be changed to 180 in near trans-sonic bullet velocities.

He has it right... Do a search for greenhill for a better understanding of bbl twist. Keep in mind always bullet length not weight. usually don't matter since a proportional increase in both but still...

 

[nickorette]

Well basicaly, I'd just like to know if you can optimize a different bullet weight for a non-matching twist of barrel.

We all know that in .223 with a 1:7 twist barrel, that 45 grain bullets will probably not stabilize too well. Basically is there any way to stabilize that lighter bullet by say, reducing or increasing velocity?

 

[CyaN1de]

Well basicaly, I'd just like to know if you can optimize a different bullet weight for a non-matching twist of barrel.

We all know that in .223 with a 1:7 twist barrel, that 45 grain bullets will probably not stabilize too well. Basically is there any way to stabilize that lighter bullet by say, reducing or increasing velocity?

Quite the opposite actually.

a 45grain bullet in a 1:7 twist barrel will OVER stabilize and possibly blow apart on it's way to the target.

Twist rate dictates the heaviest (Longest) bullet you CAN stabilize in your barrel.

For example, I have a 1:9 Twist 223 barrel that will stabilize 75grn A-Max bullets. This means that I can load any bullet under that weight/length and they will stabilize fine. I can shoot 50grain V-Max bullets out of this same barrel very accurately.

With a 1:7, you may or may not be able to shoot the lighter bullets due to increased rotation on the bullet which could cause them to fly apart.

 

[juanvaldez]

 

[nickorette]

Quite the opposite actually.

a 45grain bullet in a 1:7 twist barrel will OVER stabilize and possibly blow apart on it's way to the target.

Twist rate dictates the heaviest (Longest) bullet you CAN stabilize in your barrel.

For example, I have a 1:9 Twist 223 barrel that will stabilize 75grn A-Max bullets. This means that I can load any bullet under that weight/length and they will stabilize fine. I can shoot 50grain V-Max bullets out of this same barrel very accurately.

With a 1:7, you may or may not be able to shoot the lighter bullets due to increased rotation on the bullet which could cause them to fly apart.

Ok, so basically I'd have to decrease the power of the load in order for a lighter bullet to not over stabilize out of a fast twist barrel.

 

[Ganderite]

If your 75 gr load was accurate, you had enough twist/RPM to stabalize the bullet. If you use a lighter bullet, there is no doubt you have enough twist/RPM to stabalize, unless the lighter bullet is made of aluminum and very long.

When moving to a lighter bullet, velocity and therefore RPM increase for a bulet that needed less RPM, so you can get to the point of overstabalizing. This, initself, does not hurt accuracy, except at very long range (where you would not sue a light bullet, anyway). But the high RPM can tear the bullet apart. I have loads that do this 90% of the time. Leaves adust cloud down range, just like I smoked a clay bird.

Your 1:7 barrel will handle the VLD 80's. You would ahve to find a VLD 90 to find a bullet it might not stabalize.