Twist rate vs pressure

[Brians356]

For a particular cartridge configuration and load, does significantly increasing the twist rate (i.e. reducing the inches-per-turn ) produce more pressure (all else staying the same)?
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[chkunz]

The operative word is "significantly". You will have to be more specific to get a meaningful answer.

Even then you may only get opinion.

 

[Doug Beach]

Litz determined that twist rate did not significantly affect velocity, which suggests to me that pressure is not greatly affected.

 

[Brians356]

Litz determined that twist rate did not significantly affect velocity, which suggests to me that pressure is not greatly affected.

Bryan Litz? If he stated that it's good enough for me. But what constitutes a "significant" velocity change?
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[Doug Beach]

I don’t remember the numbers, but they were trivial. Single digits?

 

[Ned Ludd]

http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/

 

[Brians356]

http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/

I asked about pressure not velocity. There seems to be an assumption that if velocity drops somewhat from faster twist rate pressure must also have dropped. But my limited training in physics tells me that the lower velocity with faster twist must be caused by an increase in friction. Is there another explananation? And if friction has increased enough to slow the bullet measurably, it follows that the slowed bullet would actually increase pressure behind the bullet. Where's the flaw in that logic?
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[Doug Beach]

I asked about pressure not velocity. There seems to be an assumption that if velocity drops somewhat from faster twist rate pressure must also have dropped. But my limited training in physics tells me that the lower velocity with faster twist must be caused by an increase in friction. Is there another explananation? And if friction has increased enough to slow the bullet measurably, it follows that the slowed bullet would actually increase pressure behind the bullet. Where's the flaw in that logic?
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A potential flaw, is relating low velocity to increased friction. If you have a meaningfully oversize bore, friction will be lower, as will velocity. If you moly your bullets, conventional wisdom says you will reduce friction, and lower velocity.
It seems reasonable that a faster twist would increase friction, but it could also simply be that of “x” energy available, more is being spent on spin, less on velocity, while pressure remains relatively constant. I certainly don’t know.

 

[Brians356]

It seems reasonable that a faster twist would increase friction, but it could also simply be that of “x” energy available, more is being spent on spin, less on velocity, while pressure remains relatively constant.

I like your idea. That's the crux of the biscuit, as Frank Zappa put it. I still believe friction is the key here, but I have an open mind because as I said my exposure to physics is limited, but I do have a math degree so I have some chance of understanding an explanation. Where are our resident ballisticians? Did I post in the wrong forum?
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[Ned Ludd]

Friction would increase marginally with increased pitch (twist rate), all else being equal. The land profile, groove depth, and bore diameter would also be important determinants, as they all affect surface contact area (i.e friction) and/or pressure. The bottom line is that the change is marginal within a reasonable twist rate range (12 to 8) in a 30 cal barrel. Although the test showed a very slight increase in mean velocity, it was not statistically significant, being smaller than the average velocity SD value for each individual barrel. According to Litz' data, any increase in friction was more than offset by increased pressure, or velocity would have decreased as the twist rate increased. But the key point here is that this is a statistically insignificant velocity change over a pretty wide twist rate range, and it's a pretty safe bet that any changes in pressure aren't much greater, even if not equal to zero. In other words, it's not worth a great deal of concern.

 

[Brians356]

Friction would increase marginally with increased pitch (twist rate), all else being equal. The land profile, groove depth, and bore diameter would also be important determinants, as they all affect surface contact area (i.e friction) and/or pressure. The bottom line is that the change is marginal within a reasonable twist rate range (12 to 8) in a 30 cal barrel. Although the test showed a very slight increase in mean velocity, it was not statistically significant, being smaller than the average velocity SD value for each individual barrel. According to Litz' data, any increase in friction was more than offset by increased pressure, or velocity would have decreased as the twist rate increased. But the key point here is that this is a statistically insignificant velocity change over a pretty wide twist rate range, and it's a pretty safe bet that any changes in pressure aren't much greater, even if not equal to zero. In other words, it's not worth a great deal of concern.

Thanks, Ned, that resonates.
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[kvd]

Friction would increase marginally with increased pitch (twist rate), all else being equal. The land profile, groove depth, and bore diameter would also be important determinants, as they all affect surface contact area (i.e friction) and/or pressure. The bottom line is that the change is marginal within a reasonable twist rate range (12 to 8) in a 30 cal barrel. Although the test showed a very slight increase in mean velocity, it was not statistically significant, being smaller than the average velocity SD value for each individual barrel. According to Litz' data, any increase in friction was more than offset by increased pressure, or velocity would have decreased as the twist rate increased. But the key point here is that this is a statistically insignificant velocity change over a pretty wide twist rate range, and it's a pretty safe bet that any changes in pressure aren't much greater, even if not equal to zero. In other words, it's not worth a great deal of concern.

What Ned said plus the fact that to shoot barrels with different twist rates means you're shooting two different barrels so the qualifier "all else staying the same" is difficult to make real. Consequently, a statistically insignificant increase in pressure, friction, velocity or whatever may be no difference at all save for the difference between two barrels.