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Twist Rates and Velocity?

 
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  #15  
Old 03-12-2013, 11:46 AM
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Re: Twist Rates and Velocity?

Wow.... I'm not sure I want to step out in front of that debate. But there is another benefit to using a faster twist in a .308 or any other caliber for that matter, gyroscopic stability.

The standard twist will show three to four times more loss in gyroscopic stability at 1,300 meters. Which is more then 200 m into subsonic flight, but the faster twist will also get better groups at shorter distance, but not at 100m. More twist in the barrel imparts more spin on the bullet, thus more gyroscopic stability is retained through supersonic and into subsonic flight. What is lost in subsonic flight is the loss of BC due to the loss of velocity and gyroscopic stability, which allows more oscillation, which is over come by more drag, and reduced loss of BC.

So if we spin the bullet faster to fight off the effects of transonic shock waves, either through velocity or spin, the bullet will retain more BC further downrange. This results in better groups and less hold at distance. I don't really fret to much about velocity or 100 m/yd. groups to much on big game rifles/tactical/match rifles because the bullet hasn't gone to sleep yet. That and I'm more interested in sheer accuracy at distance then velocity alone, gravity and its effects on bullet drop is a constant that is easy to measure for a given speed at which the bullet is traveling from the barrel.

Honestly any small loss or gain in velocity isn't going to matter downrange on a target/game animal. The effects on group size and the benefits that could make, is what I would consider for using a faster twist rate in a barrel for a given caliber. It's a known fact that under stabilization does have an effect on stability of a bullet. Whereas over stabilization is something I've always heard of, but never seen. I don't think you can over stabilize todays bullets in rifles used for medium-large caliber weapons. But don't jump down my throat if you think I'm wrong...... just put it out there for information, so we can keep these posts on a more civil playing ground.
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  #16  
Old 03-12-2013, 12:01 PM
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Re: Twist Rates and Velocity?

According to physics, a slower twist rate should result in a greater velocity. This is assuming that the energy imparted to the bullet is constant. Bullet energy is the sum if the energy due to velocity and the energy due to spin. Faster spin means more energy is residing there, which leaves less for velocity. At the end of the day, however, the important thing is to have the bulled spinning fast enough to be stable, but not so fast that it flies apart due to centrifugal forces. I shoot a .257 STW with a 29" barrel that flirts with the latter of these. I tend to use solid copper bullets because I have seen less "premature fragmentation" from them.
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  #17  
Old 03-12-2013, 12:09 PM
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Re: Twist Rates and Velocity?

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Originally Posted by longrangehunterII View Post
Wow.... I'm not sure I want to step out in front of that debate. But there is another benefit to using a faster twist in a .308 or any other caliber for that matter, gyroscopic stability.

The standard twist will show three to four times more loss in gyroscopic stability at 1,300 meters. Which is more then 200 m into subsonic flight, but the faster twist will also get better groups at shorter distance, but not at 100m. More twist in the barrel imparts more spin on the bullet, thus more gyroscopic stability is retained through supersonic and into subsonic flight. What is lost in subsonic flight is the loss of BC due to the loss of velocity and gyroscopic stability, which allows more oscillation, which is over come by more drag, and reduced loss of BC.

So if we spin the bullet faster to fight off the effects of transonic shock waves, either through velocity or spin, the bullet will retain more BC further downrange. This results in better groups and less hold at distance. I don't really fret to much about velocity or 100 m/yd. groups to much on big game rifles/tactical/match rifles because the bullet hasn't gone to sleep yet. That and I'm more interested in sheer accuracy at distance then velocity alone, gravity and its effects on bullet drop is a constant that is easy to measure for a given speed at which the bullet is traveling from the barrel.

Honestly any small loss or gain in velocity isn't going to matter downrange on a target/game animal. The effects on group size and the benefits that could make, is what I would consider for using a faster twist rate in a barrel for a given caliber. It's a known fact that under stabilization does have an effect on stability of a bullet. Whereas over stabilization is something I've always heard of, but never seen. I don't think you can over stabilize todays bullets in rifles used for medium-large caliber weapons. But don't jump down my throat if you think I'm wrong...... just put it out there for information, so we can keep these posts on a more civil playing ground.
Thanks longrangehunter, this thread wasn't intended to be a debate. I was seeking information only and some didn't understand that or what I was seeking. You, Edd, Joe and paphil did understand the question.

Interesting theory on greater stability leading to higher BC down range. Do you have any reference for it?

I am asking the question to determine the pluses and minuses of higher twist rates. Higher twist rates stabilize longer, higher BC bullets but they also give up velocity.

The question is what is the net gain or loss. Another way of looking at it is that monometals, which I like to shoot, require more twist than lead core bullets, which I also like to shoot. If I chase the twist requirement of let's say a 9 twist for a mono in 308, when a 10 twist would work fine for a 230 hybrid, how much performance am I loosing from the 230? And is it worth it?
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  #18  
Old 03-12-2013, 12:22 PM
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Re: Twist Rates and Velocity?

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Originally Posted by andromedastudio View Post
According to physics, a slower twist rate should result in a greater velocity. This is assuming that the energy imparted to the bullet is constant. Bullet energy is the sum if the energy due to velocity and the energy due to spin. Faster spin means more energy is residing there, which leaves less for velocity. At the end of the day, however, the important thing is to have the bulled spinning fast enough to be stable, but not so fast that it flies apart due to centrifugal forces. I shoot a .257 STW with a 29" barrel that flirts with the latter of these. I tend to use solid copper bullets because I have seen less "premature fragmentation" from them.
Thanks Andromeda. First time poster? Welcome to the Forum!

As mentioned in my previous post, I like shooting both monometals and lead core bullets. And the question for me is, is the loss of performance of one worth the gain of performance for another? Subjective on my part, but is there a way to quantify or calculate the loss of velocity energy to spin energy to help in my evaluation?
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  #19  
Old 03-12-2013, 12:22 PM
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Re: Twist Rates and Velocity?

I give no consideration to the twist rate for any purpose other than bullet stability.
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  #20  
Old 03-12-2013, 02:28 PM
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Re: Twist Rates and Velocity?

This thread is proving to be helpful for me as well. I am doing a new 300RUM build in the next couple of months and was debating on how much twist to go with for the 230's. I am using a Broughten 5C of about 31". Tim North of Broughten highly recommended using a 9 twist because of the stability and accuracy they are seeing at extreme distance with this same chambering and bullet. He felt that a standard 10 twist would not provide as much stability/accuracy at extreme distance. I had a pretty reputable smith a few weeks ago recommend that I go with an 11 or 12 twist??? So obviously it's a very debatable subject. I have decided to go with Tim's recommendations and use a 9 twist even if I may be giving up a small amount of velocity.
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  #21  
Old 03-12-2013, 02:33 PM
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Re: Twist Rates and Velocity?

Hi Montana Rifleman,

The book Applied Ballistics for Long Range Shooting by Bryan Litz is highly recommended reading for understanding external ballistics at great distances, but not light reading material by anyones means.

If it were me, or someone asked my opinion about which twist rate would I choose, I'd go as fast as possible from the barrel maker for the caliber. 1:8"; 1:8.5" or 1:9" in any .264; .284; or .308 would work very well for long range work with any of the long high BC bullets, or even the lighter ones. (Read "The Future of Twist Rates" by Todd Hodnett) in this months Sniper by Shooting Times magazine. Todd explains that even with a 14" 1:8" twist barreled .308 shooting 168-grain .308 ammo was able to out shoot everyone else until they switched to 175-grain ammo to make hits that far.... so I think the higher BC bullets would work even better in these barrels at those distances due in part by the added gyroscopic stability achieved along with there high BC.

Most of this is noticed only when the distances are stretched, the high retained velocity of those heavier bullets translates into more ft./lbs. of energy on impact at those great distances. Which does help to stay within the needed energy levels (1,000-1,500 ft./lbs.) for a big game animal or a target as far as less bullet drop that would be experienced.

I have a 300 Jarrett, and wanted a 1:10" twist to use those heavy long bullets, but got a 1:12" instead because that's what Kenny uses in his 300 Jarrett. We had that debate (years ago) and I lost because he doesn't like the interior ballistics of a loaded round when the bullet is seated well below the neck. So he was doing a re-barrel job for me on a Hunter Benchrest Gun built by someone else in .308 Bear which had to have the ammo load through the magazine, and the bullets would have to be deeply seated to fit, and among other things still weigh 10 lbs. or less with the scope which it does. Kenny now does make such a rifle for hunting or target use with his 300 Jarrett, but it's a single shot gun, and wears a 1:10" twist barrel.

So the lead angles and throat length would need to be addressed with regards to OAL within a given bullet combination for the added benefit of optimum interior ballistics. Which can through a monkey wrench in the big picture depending on which bullets are used for any cartridge and whether or not if works through the magazine you are using.

Hope that helps, and please forgive me if I'm repeating something you already know? As much as I think I know a subject, I still like reading something that might help me better understand something.

Good luck and thanks
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