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Twist Rates and Velocity?

 
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  #22  
Old 03-12-2013, 08:36 PM
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Re: Twist Rates and Velocity?

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Originally Posted by Tumbleweed View Post
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.
Well I guess we'll see how they (230's) work in my Broughton 5C 10 twist 300 RUM. I'll have another big 300 in the works before long and that's one of the reasons for this thread.
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  #23  
Old 03-12-2013, 09:14 PM
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Re: Twist Rates and Velocity?

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Originally Posted by longrangehunterII View Post
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
Cole,

I have Bryan's first book but I admit I haven't thoroughly read it through. Thanks for the info. A lot of interesting stuff and it sounds like a little tighter twist might be better than hanging on the edge.

Would still like to know if there's a way to calculate velocity loss with faster twist.
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  #24  
Old 03-12-2013, 11:09 PM
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Re: Twist Rates and Velocity?

On another note about barrels, a few will never flute the barrel, and long ago Kenny Jerrett and I had this conversation, and a few other barrel makers. But don't flute the barrel! I can understand a gun that was made and way to barrel heavy, and a error was made. So to correct that issue the barrel was fluted, but before you make that mistake, or someone else, fit the gun with a barrel that has the proper contour for the application. Lose the weight with a shorter barrel, which is stiffer, or a smaller contour if need be. Either way in the end it will be better. Flutes look cool, but they unfortunately don't heat evenly. Plus all that machining work cutting those flutes is done after all the important stuff making the barrel. Why produce a perfect barrel lapping the interior to proper dimensions only to undo that work later on with flutes cut down 8/10 of the entire length of the barrel. I wondered about this myself but I am by no means an expert in metallurgy or the effect of machining on metallurgy for that matter but after a lot of reading and discussion it made sense.

Accuracy International LTD did a test using three laser devices on the scope, action, and the barrel. They set the lasers to the same POI, and after firing found that no matter which fluted barrel they used, the dots from the scope and action stayed in place, but the barrel's laser would manifest a POI shift when heated. While the POI shift from the warming barrel greatly diminished when they used barrels w/o flutes! Which over a decade ago I heard this from Kenny and a few others. I would have thought more people would have realized this same observation, but I see more gun barrels with flutes then without these days. Make no mistake about it, the goal is to mitigate the effect of heat for accuracy during fire, fluting exacerbates it!

Those that read this and have fluted barrels ( I have two out of a dozen that don't) I'm sure will say differently, I'm only stating what I've either read or been told by those that make barrels for a living, and the facts speak for them selves. A gun used for hunting big game, may only require one (1) round or three (3) to bring an animal down. So fluting isn't a big deal, but I also have had some that were fluted that didn't shoot to my expectations and pulled them. The diameter of the barrel is what's important for stiffness to reduce the effects of harmonics.

Besides, the money someone would spend having the barrel cut with flutes could be put to better use for something else?
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  #25  
Old 03-13-2013, 01:38 AM
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Re: Twist Rates and Velocity?

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Originally Posted by MudRunner2005 View Post
You misunderstood....The bullet SPINS faster from a longer barrel BECAUSE there is more time it is touching the rifling and being acted upon by the rifling force for stabilization purposes. .
No, say a bullet is shot from a 16.5" barrel at 2700 fps. 10 " twist
Or a 26" barrel at 2700 fps 10" twist. Its still going to be doing 1 revolution every 10"
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  #26  
Old 03-13-2013, 03:54 AM
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Re: Twist Rates and Velocity?

I can add another couple of data points for you regarding twist and effect on velocity.
From the empirical side, I'm a Service Rifle competitor and we're shooting 20" AR's in 223 Rem with fast twists (1 in 8 or better). I've played with 9, 8, 7 and 6.5 twist barrels shooting pretty much the same loads (77 gr Sierra at about 2675 to 2700 over a chrono. In my experience there wasn't a difference I could see attributable to the twist.

From the theoretical side, Bob Rinker addressed this question in his book "Understanding Ballistics". He calculated the amount of energy required to spin a bullet and came to the following conclusion:
Quote:
...That is a lot of mathematics and work to show and prove the energy required to rotate the bullet is very small. Extremely small. If an example is worked out, it will fall in the general area, depending on cartridge, twist, etc. of between 1/4% and 1% of the available energy. The loss in velocity, compared to a barrel with straight rifling (no twist) would be even less.
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  #27  
Old 03-13-2013, 08:31 AM
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Re: Twist Rates and Velocity?

One other thought on spin and stability. Denser air requires a faster spin. a marginal spin at 7000 feet elevation may go unstable at sea level when shooting at a 1000 yard or farther target. Go with the mfg guidelines. If Walt Berger says you need a 1/9 for a bullet, I'd go with a 1/9. Most bullets will be stable for short distances ( less than 300 yards) regardless of the twist rate. Try shooting some 80gs in a 22-250 with a 1/14 twist. I tried them a while back, not sure where they went I really don't think the faster twist reduced my velocity very much, but then once I could shoot the heavier bullets, I quit using the lighter ones.
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  #28  
Old 03-13-2013, 09:23 AM
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Re: Twist Rates and Velocity?

IMO, twist effects velocity by resisting travel down the bore.
It takes more energy to push a bullet in a 10 twist than it would in a 20 twist.
In "general" that creates higher pressure due to more bore resistance which MAY increase velocity.
On the other side, if you are at max pressure then you may have to reduce the load and go the same or slower with a faster twist.

Also IMO, a bullet should be shot at the slowest twist for ALL conditions you will face.

First, bullet RPM slows down at a much slower rate than forward velocity so it effectively becomes more stable the longer it fly's;

Second, no bullets are perfect.
While in the bore a bullet rotates around the centerline of the bore.
When the bullet emerges from the muzzle it rotates around the bullets center of gravity.
This "jump" to the CG is dependent on the difference between the CG and the centerline of the bullet formed in the bore AND velocity.

There is an article on the Lilja website on this:
Lilja Precision Rifle Barrels - Articles: A Look at Bullet Imbalance and Twist

Also there was something on the web that I can't find right now that experimented with machining an imperfection in a group of bullets to create an imbalance.
Then they indexed and shot the bullets in 90 degree increments and the bullets all grouped to the same 90 degree group as predicted:







edge.
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