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Twist vs Bullet Weight?
I have a custom Mauser rifle in .308Win., with a 22" barrel. Twist in the rifling is one in ten inches. What bullet weight range would give the best accuracy in this barrel?
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RE: Twist vs Bullet Weight?
Sounds like the typical twist rate for a .30. Typical .308 bullets from 150-180 gr should work well.
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RE: Twist vs Bullet Weight?
The 1 in 12 is ideal for 168gr bullets. The 1 in 10 is ideal for heavier bullets like 175gr of 180gr. But really, you could shoot any bullet from 150gr to 200gr in either.
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RE: Twist vs Bullet Weight?
Both my M70 .308 like 165gr.
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RE: Twist vs Bullet Weight?
Good rule of thumb, tighter the twist, heavier the bullet...basically, the lighter the bullet, the slower it's going, so to get the same RPM's it would need a tighter twist rate.
On your .308, 1:10" is pretty tight, but it's not overly so, your rifle should shoot very well with 175grns and 180s, but there's no reason it won't shoot very well with 150's or 130's even. I've got a 1:10" that I run 100 and 110grn (.30-06) bullets through, at max velocities they shatter at leaving the muzzle, slowed down a bit I get about .5MOA accuracy to 200yrds, 300 on a calm windless day (which happens about once every other leap year in KS), it also shoots 180 and 200grn bullets VERY well, the velocities are much lower than I can push those 100's to, but like I said, I must limit their speed. |
RE: Twist vs Bullet Weight?
The faster a bullet is traveling the less twist you need in a barrel. The LONGER a bullet is the more twist you need in a barrel. Twist rate depends on the velocity at which the bullet is traveling, the LENGTH of the bullet, and the bore diameter. Basically a 1-10 .308 will stabilize up to about a 200 grain bullet sufficiently. However there is a minimum velocity that if the bullet drops below it will become unstable so you need to ask yourself how far you will be shooting and what velocity will the bullet be travelling at that distance.
This is how to calculate what twist rate a given barrel needs with a given bullet length. I'm quoting myself from another thread having to do with calcualting a twist rate needed for a given bullet. What you need to do is refer to some reloading manuals and other charts to figure out how fast a given length bullet will travel with a given barrel length. A bullets velocity will change about 30 fps per inch of berrel added or removed when the bullet is traveling between 3000 and 3500 fps. So from a reloading manual you can get pretty close in figuring how fast a given bullet will travel with a given barrel length. Measure how long the bullets are that you might be using and go to the calculator at the link and you can figure out how the minimum velocity for each given bullet. Then take this information and refer to your reloading manuals to figure out how long of a barrel you would need to achieve a certain velocity. EXAMPLE: The barrel is 1 in 12 inch twist rate. The bullet is .723 inches long and the bore is .224 inches. Plug in those numbers and you get a minimum velocity of 2441 fps. to stabilize that particular bullet. You can get that much velocity out of a revolver. A 1 in 12 twist barrel will stabilize bullets up to about .8 inches long at 3000 fps. and that is a heck of a long .224 bullet. http://www.uslink.net/~tom1/twistrate.htm |
RE: Twist vs Bullet Weight?
Greenhill Formula:
minimum required twist rate = (150 * bullet_diameter2) / bullet_length |
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