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Variable barrel twist.

 
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  #1  
Old 03-05-2010, 04:23 PM
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Join Date: Mar 2010
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Variable barrel twist.

Can anyone point me to an article or information on the advantages and disadvantages of a rifle barrel cut with a variable twist, slow to faster. I can see a possible disadvantage at least in the manufacturing phase, but I'm not sure of any advantages. How much of barrel twist is required for proper bullet stabilization? In other words, if I have a 30-inch .30 cal. barrel, and the first 20-inches were smooth bore and the last 10-inches were1/10 twist could I impart enough spin to the bullet to stabilize it.
If I have shown my ignorance of the law of physics in anyway, I apologize before hand.
Thanks.
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Old 03-05-2010, 08:07 PM
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Re: Variable barrel twist.

You won't want a long stretch of smoothbore, usually called freebore, because accuracy would probably be bad. Also when the bullet finnally hit the rifling at 1500-2000 fps, it would be hard on the jacket. Gain twist is used when a very fast twist is needed for a very long bullet. Some of the bullets being tested now are 7 calibers long, 7 x .338 = 2.37 inches long. Even in 338 this might need a 5 or 6 twist. You can't start at 5 or 6 so they might start at 10 and finish at 6.

I've read that anything faster than 7 twist at the start and the bullet may skid rather than take the rifling as it should. But this may be caliber dependent and is certainly somewhat dependent on bullet type and design.

Many of those really long bullets are monometals and have driving bands.
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Old 03-06-2010, 10:41 AM
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Re: Variable barrel twist.

I have been told that the main advantage to a gain twist comes from imperfect (non uniform) twist process. i.e. a little faster at the muzzle is much better than a little slower. If the twist is perfect all the way through the bore, there is probably nothing to be gained by a gain twist as far as accuracy goes. There MAY be some minimal gain to velocity by starting slower as pressures may not build as fast but the gain is so negligible that it is probably not worthwhile. Talk to Frank at Bartlein barrels. He is very knowledgeable and can probably answer all your questions. I think what he will probably say is the jury is still out.........Rich
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Old 03-06-2010, 03:51 PM
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Join Date: Mar 2010
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Re: Variable barrel twist.

Thanks to everyone for the info and their insights. I had not thought of the problem of bullet jacket material hitting the rifling after a good running start.
I might add, that I have no intention of having a rifle with free bore or a variable twist built. As I said in my profile, I'm 72, and not planning on adding anymore hardware to my collection, but have always been interested in firearm equipment and techniques, that would allow long range accuracy. Thanks again
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Old 03-06-2010, 03:57 PM
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Re: Variable barrel twist.

Gain twist barrels were used in some muzzle loaders over 150 years ago. They have been plaied with and talked about ever since, and since no one in the bench rest comunity is even talking about them any more I would doubt that there would be any thing to gain in the way of accuracy.
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Old 03-06-2010, 04:45 PM
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Re: Variable barrel twist.

Quote:
Originally Posted by Catfish View Post
Gain twist barrels were used in some muzzle loaders over 150 years ago. They have been plaied with and talked about ever since, and since no one in the bench rest comunity is even talking about them any more I would doubt that there would be any thing to gain in the way of accuracy.
Actually, there are a few BR guys using them now but I think for 100 yd. shooting not LR........Rich
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Old 03-06-2010, 04:56 PM
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Re: Variable barrel twist.

Quote:
Quote:
Originally Posted by Catfish View Post
Gain twist barrels were used in some muzzle loaders over 150 years ago. They have been plaied with and talked about ever since, and since no one in the bench rest comunity is even talking about them any more I would doubt that there would be any thing to gain in the way of accuracy.
I think the biggest advantage with gain twist barrels is for use with extremely high BC bullets requiring extremel tight twists. You dont find BR shooters, shooting many of these typse of bullets.

The idea makes sense to me, to gradually increase the twist of a bullet requiring a very tight twist, especially at high velocities. I couldn't say for sure, but it seems it would keep pressure down and would reduce stress on the bullet or keep it from slipping against the lands.

-Mark
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