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Twist rate vs velocity

 
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  #1  
Old 05-19-2009, 09:17 PM
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Twist rate vs velocity

Anyone out there quantified or have a source that has documented what the velocity differences are in different twist barrels? Is there a rule of thumb?
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Old 05-20-2009, 09:55 AM
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Re: Twist rate vs velocity

I don't see how this could be reduced to a rule with any accuracy.
Afterall, there are so many other variables affecting velocity along with twist..

One cal and cartridge might react to twist differences to a greater/lessor degree than the same cal in another cartridge(or even another barrel of same twist). Different powders and system pressure curves..

So it wouldn't just be a matter of adding/subtracting energy converted to torque.
I don't even know that you could predict whether velocity would go up, or down, when comparing 6ppc -vs- 6-06 for different twists. The powders and pressure curves(and usually barrel lengths) are so different.
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Old 05-20-2009, 10:10 AM
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Re: Twist rate vs velocity

Just too many variables for anything closely resembling a rule IMO.

regular vs canted rifling vs gain twists
freebore and lead variances
Bore dimensions (tight vs standard bore)
diameter of bullets (fat vs thin for caliber) Hornadys are known for being "fat"
primer variances effect both MV, SD/ES and pressures greatly

BH
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Old 05-21-2009, 12:38 AM
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Re: Twist rate vs velocity

Mike and BH are spot one here. Too many other variables come into play.

The accepted theory is that all else being equal, a tighter twist barrel will produce slightly higher pressure than a slower twist barrel. This is straining gnats though.
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Old 05-21-2009, 11:04 AM
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Re: Twist rate vs velocity

Twist is all about stabilizing bullets. The goal is to have the least amount of twist that will stabilize the bullet you use. Twist rates have to do with bullet length, not weight, although they are generally related. However, one can get by with a slower twist if the velocity is increased, since the RPM of the projectile is what does the stabilizing.
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Old 05-21-2009, 02:53 PM
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Re: Twist rate vs velocity

I don't know if Katbird is addressing your question, but his 'twist understanding dump' should really be corrected/enhanced..


Quote:
Originally Posted by Katbird View Post
The goal is to have the least amount of twist that will stabilize the bullet you use.
My goal is to FULLY stabilize bullets, as soon as possible.


Quote:
Originally Posted by Katbird View Post
one can get by with a slower twist if the velocity is increased, since the RPM of the projectile is what does the stabilizing.
This is only a general rule, in a very limited sense. I'm sure many have screwed themselves with this notion.
RPMs do go up with velocity, and stability usually goes up with velocity(a very tiny amount), but not necessarily.
It depends on where the velocity puts you on that particular bullet's drag curve.

Reason being, stability is tied to displacement per turn. It's the upsetting quantity for each turn to overcome. With this, bullet twist requirements are expressed as such (like 8"per 1 turn) based on 8" of standard atmosphere displaced, and the drag coefficient of that bullet, at chosen velocity, per 1 turn.

There are other physical attributes of bullets affecting stability, but TIME never comes into play.
Stability doesn't really follow turns per time(RPMs). If it did, bullet stability requirements would be expressed in RPMs,,, and none are.
They never will ever be, because a time based definition of one condition would fail many tests elsewhere.
-For instance, a bullet barely stable at 80deg, can whack paper sideways at 40deg. Same velocity, same RPMs, but different displacement(denser atmosphere).
-In contrast, an unstable bullet at 1,500fps MV can punch round holes at 2,500fps. A huge improvement, but the bullet is far from fully stable, and would likely perform poorly. It can be the 40deg mishap above in waiting.
So velocity helped, yet it would take an impossible 15,000fps MV to fully stabilize this bullet, with this twist and condition.
So why such a gigantic change in RPMs to improve stability(from 171Krpm to 1029Krpm)? It's because RPMs never mattered..
I used 10.5:1 twist for this test and whether 1,500fps or 15,000 the displacement was still 10.5" per turn. The difference, all along, came down to atmosphere and the bullet's drag curve.
I could shoot it FULLY stable, at 2500fps, with use at 90deg and 6000' above sea level. Or I could choose a 9.5tw barrel from the beginning for use at 90deg at sea level. Or I could choose a 9tw barrel and always be fully stable.

I'm no expert, so you might run tests for yourself at JBMs.
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Old 05-21-2009, 04:01 PM
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Re: Twist rate vs velocity

It does not seem based on all the responses like velocity is a consideration if one is on the fence trying to decide between two twist rates.

I would think if the same reamer is used on several identical barrels (minus the twist rates) many of the variables at play could be eliminated/minimized. These barrels could be tested with several different loads made with indentical loading procedures. Pressures and velocities could be recorded to determine their relative performance.

I wondered also if one would find that on average max charges may increase with the slower twist rates while pressures remain equal to the faster twist barrel. It would be fun to test but I sure would not want to foot the bill for that much testing.

My thoughts on twist and velocity were not really the torque losses on the bullet but the additional bearing surface created by faster twist rates and possibly more importantly the angle of the rifing impeding the forward motion of the bullet. In theory At some point the rifling would stop the bullet completly since the friction created by the rifling would block any movement kind of like a threaded bolt.

I am not trying to be arguementative and will accept the experiece of others that velocity is not affected by twist rate. I just wanted to put my thoughts out there I guess.

Last edited by 3006savage; 05-21-2009 at 09:42 PM. Reason: added not.
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