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Thread: Rate of twist

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    Moderator hunt_ak's Avatar
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    Default Rate of twist

    Been trying to learn about this aspect of rifling and was wondering if anyone could give me a lesson on twist-rate. I have been hearing that a 1 in 9 gives you a wide range of bullet size to range accuracy but didn't know this is an accurate statement. Also, RH twist vs. LH twist?

    Any an all info!!

    Thanks

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    Quote Originally Posted by hunt_ak View Post
    Been trying to learn about this aspect of rifling and was wondering if anyone could give me a lesson on twist-rate. I have been hearing that a 1 in 9 gives you a wide range of bullet size to range accuracy but didn't know this is an accurate statement. Also, RH twist vs. LH twist?

    Any an all info!!

    Thanks
    Your question is incomplete. In what, for what?

    The twist rate required to stabilize a bullet in flight (which is different than stability in animal tissue) depends on several factors.

    The length of the bullet: Longer bullets require a faster rate of twist.
    The velocity of the bullet: Higher velocity calibers do not require as fast of a twist rate as slower bullets.
    The diameter of the bullet: Larger caliber (diameter) bullets can be stabilized at a slower twist rate than smaller diameter bullets will need.
    The construction of the bullet: To a lesser extant, this matters. The formula is to calculate for lead bullets through air. Aluminum bullets would need much faster twist rates. If we shoot through mud we would need a faster twist rate. Also there is a limit on how fast we can spin a bullet before it self destructs in flight.

    The term for bullet stability is surface feet per minute. That is, rotational speed times the circumference of the spinning projectile. (rpm x c.) Velocity and twist rate will determine RPM.

    It doesn't matter whether it is right or left twist and no we don't need left hand twist in the southern hemisphere.

    A longer bullet, not necessarily a heavier bullet, requires more twist. A .264" bullet of 140 grains weight will need more twist than a 140 grain .308" bullet because it is longer, if launched at the same velocity. (Of course there is a limit on velocity about everything is about 3000 fps-ish)

    The formula to calculate the needed twist rate is called the Greenhill formula. This will give the minimum twist needed to stabilize a projectile through air for most applications but other factors come into play.

    For a hunting bullet, we generally use a soft point expanding bullet. When the bullet impacts it expands and shortens. This keeps the CG forward and stability is maintained for some time in animal tissue. This helps to make for straight line penetration. When a bullet becomes unstable after impact it won't travel in a straight line. A solid or an FMJ bullet that doesn't expand will need greater twist rate to stabilize in animal tissue for straight line penetration. A spitzer bullet that does not expand will be very unstable at impact and will travel a curving course through the target animal and may miss the vitals. We really cannot spin a bullet fast enough to stabilize a spitzer in animal tissue, unless it expands quickly.

    If we shoot solids we could use lighter weight, and that would make them shorter and by being lighter they could be driven much higher velocity and that would give the needed surface feet per minute to stabilize.

    Blunt nosed, large diameter, heavy bullets don't need much twist to be stable after impact and these bullets will consistantly penetrate very well.
    (Heavier here adds momentum which helps push the bullet deeper.)

    Small caliber, light weight spitzer bullets are at the opposite end and do not penetrate worth a crap. (That's a technical term.)

    What else do you want to know about doing the twist?
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    Moderator hunt_ak's Avatar
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    Quote Originally Posted by Murphy View Post
    Your question is incomplete. In what, for what?
    Murphy, thanks for the reply! Very informative...

    Yeah, I knew it was going to be. I guess thats why I put any and all info welcome!

    I was reading about/listening to a program about Stag's new 6L Super Varminter and the president of the company said they normally deal in 1/9 twist and that gives them a good "compromise" with different bullet lengths when dealing with the 5.56 (although I believe their new SV is a 1/8).

    So shooting longer range with with longer bullets (same make/model/caliber), a faster twist rate is needed for stabilization over the entire length of flight?

    How do you determine twist rate for rifling if it is not written/stamped on the firearm anywhere (or is it and I just can't find it)?

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    To determine optimum barrel twist rate for a specific caliber and bullet there is a web site that will calculate it using the greenhill formula. www.z-hat.com/twistrate.htm

    I think the best way to determine what twist rate you have in a particular rifle is to call the manufacture.

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    Using a cleaning rod, patch, oil, bristle-brush and tape measure: make a mark on the cleaning rod, insert an oiled patch wrapped around the bristle-brush, with a tape measure laying alongside the rifle, slowly insert the cleaning rod allowing it to rotate. When the mark comes up again, note on the tape measure how far you've moved.
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    Thanks from me too, Murphy.

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    Quote Originally Posted by hunt_ak View Post
    Murphy, thanks for the reply!
    So shooting longer range with with longer bullets (same make/model/caliber), a faster twist rate is needed for stabilization over the entire length of flight?
    Yes, also, bullets that have better flight characteristics are longer (heavier=higher SD=better BC) and need more twist. They need better BC to keep velocity above the speed of sound as sub-sonic projectiles need different shape/spin to be stable.

    If you are not shooting 600 yards or more this is just trivia.
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    So bascially fussing with rate of twist under ~500 yards is not necesary?

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    Quote Originally Posted by hunt_ak View Post
    So bascially fussing with rate of twist under ~500 yards is not necesary?

    Well, no, not necessarily. Twist rate for a long bullet for the long line, for some calibers, may over stabilize the bullet for the best possible 100 to 200 yard group.

    A prime example; The Sierra 155 grain HPBT Palma bullet with a one in ten twist 308 Winchester will group sub .5 MOA at 800 yards but will shoot about 2 MOA at 100 in some guns (longer barrelled, Palma type, higher velocity rifles). It will shoot very well from 16" barrels (lower velocity) and group the best of any bullet for the 308 at 200 yards. When a bullet is fired it has a violent beginning and will yaw (wobble) for some time until it stabilizes (goes to sleep) then it will hum along quite nicely. In a 1 in 12" twist the Palma makes a very good 100-200 yard standard hunting rifle target bullet. It takes the combination of twist rate and velocity to achieve the stability at the long line for this bullet. It was designed to get this from the 308 if the barrel is 29" long and the twist is 1 in 10".

    For the 5.56mm and its various bullets and with its various barrel lengths the 1 in 7 twist will fit the requirement of the M855 steel rod core bullet in short barrels (lower velocity) at 500 yards. We do not need a 1 in 7" twist for; shorter bullets (all lead core, lighter weight), standard barrel lengths, or short line shooting (less than 400 yards) Contrary to popular belief, a faster twist does not make a more accurate rifle, quite the opposite in some cases.

    So many 223 shooters think that the have to have a 1 in 7" twist or they cannot hit a barn from the inside. I have shot several 3" 600 yard groups with the 223 with a 24" 1 in 9" twist barrel with 68 grain lead core hpbt match bullets. The faster twist will yield less velocity with the same pressure than a slower twist, (varies from barrel to barrel) the faster twist barrel will also wear faster and the faster twist will not group as well at the short line.

    The plain truth of the matter is that a 22 caliber bullet is not even close to the most accurate caliber. 6mm or 6,5 mm calibers will always out perform it short or long line, wind or calm and they can be had in the same platform (rifle type, action length, ad infinitum). The 308 is also an inherently more accurate caliber over a much broader spectrum than the 223 but it's recoil makes it a more difficult rifle to shoot long matches (many rounds) than the lighter calibers.

    I have no idea what you are after here and if it just information, that is fine, but I hear 223 shooters who have never fired a sanctioned match or even at a range greater than 100 yards tell me how great their little gun is and how accurate it is out to 1000 yards, or how their 6.5-24x scope will put them on target from the muzzle to the moon with no hold over. I've been doing this a long time, I've run with the best shooters in the world, I've shot hundreds of long and short line matches with dozens of different guns. I'm also an educated engineer and a boni fide gun nut. It has been my life, my hobby, my passion and my pursuit of excellence. Talk is cheap, nothing matters on match day except small groups in the center of the target. X count, is the name of the game. I've shot tighter groups at 600 yards with an iron sighted M1A than most of todays hot shot shooters can shoot with their scope sighted 223 AR rifles, at 100.

    Sorry for the tirade but I need to do that from time to time.

    Anyone else want ot twist?
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  10. #10

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    not twist related...but I have seen a person just hammer the 10 ring at 600 yds with a Savage 99 308 Lever gun from the sling and a low power scope. I have seen another person never put one on paper with a tricked out 223 bolt gun, "the" ammo, magic bullets and moon scope from a rest.
    I think our culture puts more value on the equiment specifications rather that mental and physical conditoning and shooting practice.

    Karl did not know that his Savage was not supposed to do that and the 223 shooter expected the rifle to do all of the work.

    Concur w/Murphy.

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    Quote Originally Posted by Murphy View Post
    I have no idea what you are after here and if it just information, that is fine,
    Murphy, I am indeed here, like many, to try and tap the wealth of knowledge on here. I am also looking at getting a varmit gun that i can shoot long distance (3-4-500 yards), because I think it would be fun to learn how to do these things, but also I might want to hit a prarie dog, coyote, etc. My brother has a .22-250 but I was looking into other options for something like this and was trying to dig into the 223 as an option.

    From what I've read and from what I think I want, the .308 is one of if not the best for all around performance with long range shooting. I am intrigued with the AR platform and getting one soon may be on my list of things to do because one may not be able to buy one in the near future (with the way politics may be shaping up...but thats a different story) and Stag is the only one that makes the left-handed model.

    Basically fun factor+decent long range shooting+ammo economics is why I was asking about the 5.56, but I may have to dig a little deeper...

    Thanks Murph

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    You can't beat the 223 for what you want to do. In a varmint gun it has almost no recoil, and it's cheap to shoot. In a good gun it's extremely accurate to 600 yds. I know folks with fast twist (1-8) 223 ackley improveds that shoot the 75 gr a-max to 1000 yds.

    If you're having the gun built, I'd go with a 1-8 twist, as it will work well with 50-75 gr bullets. If you get a factory gun with the common 1-12 twist you'll pretty much be stuck with 40-55gr bullets.

    I'd stick with the bolt guns as they are more flexible for handloading, you don't have to chase brass, and they tend to have a slight accuracy edge over the AR's.

    I picked up just such a gun last year at one of the gunshops. Got it for $450, sold the tasco target scope for $150 on e-bay, and put a leupold 3.5-10 vx III w/ target turrets on it. I shoot 50 gr v-max out of it. Once I get my chrono fixed and work out the drop chart, it'll be a simple task of laser target, dial in drop, estimate wind and learn the long range skills. Eventually I'll replace the barrel with a 1.-8 twist and have it chambered as a 223 ackley so I can shoot the 75 a-max. I'll also put it in a McMillan A5 stock.

    You simply can't beat .223 for what you want to do.

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    This is the rifle I'm looking at now. 1-8 twist. Obviously not a bolt gun but they gaurantee 1/2 minute @ 100 and 1" @ 200 outta the box. Chase brass??

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    Looks nice! I have a similar set up and it shoots great. One can get a device that drops brass at your feet that snaps on the receiver from Creedmore.
    The self loaders require quite a bit more prep time reloading, examination of brass, more trimming, looking for dents in the body and case mouth, extractor groove lengthening, paying attention to head space of your particular rifle.
    Not a big deal unless you are shooting mass quantities. If you are shooting a lot, you have to weight all that against just shooting the brass once. AR's are not as bad as the M14 in that regard.
    The bolt guns are much easier to load for, and 100 rds should last a very long time...

    Get one of each!

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    Quote Originally Posted by hunt_ak View Post

    This is the rifle I'm looking at now. 1-8 twist. Obviously not a bolt gun but they gaurantee 1/2 minute @ 100 and 1" @ 200 outta the box. Chase brass??

    I lost this thread for a while.

    I don't want to discourage you from the well tuned 223 in the AR rifle. The 223 chamber is a more accurate chamber than the 5.56 (no they aren't the same) and what you're showing here will be a fun gun to shoot. STAG is very good equipment and will give very good accuracy with a wide assortment of bullets. All of this is a compromise and we don't want to get too specific when it will be for general use.

    I think you'd be happy with the level of accuracy from such a gun and still have plenty of fun, inexpensive shooting. The 223 is the least expensive rifle caliber.
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    Murphy, can you give me some differences and why the 5.56 and the .223 are not the same. A guy I work with was telling me the .223 was the civiliam version of the 5.56. I told him he was wrong but didn't have any rock solid answers for him (other than something I've read of pressures and SAAMI specs I believe)

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    A good way to figure the required twist rate; (And it does not work all the time) is:::::

    The Caliber in inches, multiplied by :

    x30 for very heavy LONG FOR THE CALIBER bullets. (ie .308 x 30 = 9.24 inch twist for heavy slugs)

    x33 for medium to heavy bullets (.308 x 33= 10.1 inch twist Standard US)

    x36 for light to medium weight bullets (.308 x 36= 11 inch twist TIKKA)

    x40 for lighter weight bullets. (.308 x 40 = 12.3 inch twist)


    This seems to work with most calibers:
    .264 (6.5mm) x 30 = 7.92 inch twist Close to your 8 inch twist for your 6.5x55.

    .223 x 33= 7.3 inch twist... So it is close on most calibers.
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    Quote Originally Posted by hunt_ak View Post
    Murphy, can you give me some differences and why the 5.56 and the .223 are not the same. A guy I work with was telling me the .223 was the civiliam version of the 5.56. I told him he was wrong but didn't have any rock solid answers for him (other than something I've read of pressures and SAAMI specs I believe)
    I would have to say that the 223 is the civilian version of the 5.56x45...but it does not have the same dimensions. The dimensions for the 223 (SAAMI) Chamber is smaller than the 5.56 and the 5.56 throat is longer. I think the body and neck diameter are both larger for the 5.56 and headspace for the 5.56 is longer (the base to shoulder datum point). Thoat and leade dims and angles are also different.

    What this means is that shooting 223 ammo in a 5.56 chamber is probably ok. but shooting milspec 5.56 ammo in your 223 chamber would boost pressures significantly. I know people shoot this ammo in 223 chambers all the time but the pressure is still higher and it is not a good practice.

    Some AR makers make 5.56 milspec chambers and some use match 223 chambers and these makers usually advise against the use of milspec ammo. All of this applys to the 7.62x51 and the 308 Winchester, also, but the differences do not create the excess pressure of the 5.56 vs 223.
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    Do you have accuracy issues shooting .223 out of a rifle chambered in 5.56?

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    It may simplfy things some to say that the diamensions of the case are the same between .223 Remington and 5.56mm NATO.
    What differs are the way that rifles are chambered for those two rounds.
    Plus there are differences in the way that the cartridges may be loaded with varyuing tolerances to met the needs of hunters / shooter vs bulk military ammo production. Murphy explained the way that Civilian vs Military chambers and frre-bore works out.

    Forgive the typos I am on a conf call..
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