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Thread: rifling twist

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    Member anonymous1's Avatar
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    Default rifling twist

    I inherited a .220 swift "bullpup". It was my uncles favorite varmit gun who was a fabled long range shot with it. He passed away forty years ago and my aunt left it to me when she passed last year so there is no one left who knows about it.
    I have been reading up on the caliber and specs and want to restock it and start using it.(I`m a lefty) it has a right hand stock with a thumb hole that I can`t shoot.

    Anyway I`ve read shorter twist is better for accuracy but harder on the barrel. Can one just peer down the barrel and count the twists or is there a better way. I do not have it here with me but have been thinking about it for when I get back to it

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    Member Chisana's Avatar
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    Default

    One way to get a rough measure of rifling twist is as follows:

    1) Remove the bolt
    2) Take a high quality cleaning rod that swivels well (Dewey) and insert it into the barrel from the breach with a jag and tight fitting patch.
    3) Place a small mark on the rod near the handle, this mark will be what you watch for rotation.
    4) Place another mark on the rod where it begins to enter the action.
    5) Push the rod through the bore allowing it to swivel freely, stop when your first mark has made one full rotation.
    6) Place another mark on the rod at the point where it now enters the action.
    7) Measure the distance between the second and third marks on the rod. This will be the amount of distance traveled during one rotation.

    If this is an older rifle it will probably be a 1-13" or 1-14" twist suitable for 45-55 grain bullets. Heavier 22 caliber bullets and the associated fast twists are a fairly recent phenomenon.

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    Default twist

    Thanks chis
    sounds easy enough. .22 seemed like a pretty small hole to peer into. The story goes that he turned his own barrel so will be interesting to see what I got.

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    Twist rate is probably one of the least understood technicalities of the gun world.

    Generally the 220 Swift is 1 turn in 14". I have shot a bunch of 70 grain Sierra hpbt spitzers into 1/2 with several 1 in 14 twist 220 Swiftand 22-250's.

    Bullet stabilization needs are such that as velocity goes up twist rate can go down (slower) Swift and 22-250 are capable of launching 55 grains at 3900 fps, that's why there twist rate is 1 in 14 not 1 in 12" or 9" or 7".

    The stability of a bullet is achieved by a certain rotational velocity related to caliber technically, surface-feet-per-minute. The need of spin is based on the length of the bullet, not weight, though obviously the longer bullet will be heavier given the same caliber. The length of a bullet expressed in calibers is used to determine the twist rate needed. The Greenhill formula has been in use for over a century and is available in some of my posts here and elsewhere so I won't go into it but just wanted to cear up some misconceptions about twist rate.

    Faster twist rates do several things. Increase pressure and recoil. (you won't notice it in a 22 caliber.) Generally wear faster expecially at the throat, though this is very minor and offer futher stabilization needed for longer bullets.

    Try this. http://forums.outdoorsdirectory.com/...ight=Greenhill
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    A good web page that explains the Greenhill and other formulas for the twist needed to stabilize.

    http://kwk.us/twist.html

    One thing mentioned sometimes is over stabilization of bullets, leading to reduced accuracy. http://www.riflebarrels.com/articles...ance_twist.htm You will notice this does not effect accuracy that badly.

    Another danger is spinning the bullet so fast it comes apart. Sounds crazy but a bullet at 3900 fps out of a 1:14 barrel is turning at 200,000 rpm, and out of a 1:7 at 400,000 rpm.

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    Member anonymous1's Avatar
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    Default thats spinnin

    Wow thats amazing rpm. Now its got me wonderin as I recall many a long night as a boy listening to my Pops and Unc talkin about reloading and the Swift`s bullets not making it to the target I wonder now if it was the twist or the fps
    thanks for the info and the links good stuff ,can`t wait to get home and do some checkin

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    Quote Originally Posted by bandhmo View Post
    A good web page that explains the Greenhill and other formulas for the twist needed to stabilize.

    http://kwk.us/twist.html

    One thing mentioned sometimes is over stabilization of bullets, leading to reduced accuracy. http://www.riflebarrels.com/articles...ance_twist.htm You will notice this does not effect accuracy that badly.

    Another danger is spinning the bullet so fast it comes apart. Sounds crazy but a bullet at 3900 fps out of a 1:14 barrel is turning at 200,000 rpm, and out of a 1:7 at 400,000 rpm.
    Yeah nice job on the math there. That formula is 12/twist * V* 60 = RPM
    12/14 =.8571428 *3900 = 3,342.8569 * 60 = 200,571 RPM

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    Quote Originally Posted by Murphy View Post
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