Rifle's rate of twist
I would like to know more about the significance of the rate of twist in riflings. For example,a 1 in 9 vs. a 1 in 12. I realize it affects bullet stability in flight,but in what circumstances would one be better over the other?
Randy Summers
Randy Summers
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Each bullet length has a Minimum, Optimum, and Maximum rotation rate (RPM). The shorter the bullet the slower it needs to spin (IE: a round ball, with it's 1:1 Length/diameter ratio, is happy with 1 turn every 72").
The RPM for a given rifle/loading comes from the MV and twist rate. To slow a RPM and you will get keyholeing, to fast and the groups won't "settle down" (stop wobbling around it's flight path) until the bullet has gone a couple hundred yards down range.
If your going to be shooting short (lighter) bullets, the slower rate will be fine, on the other hand if your going to be shooting long for caliber (heavy) VLD style bullets, than a fast rate is required.
Over spun is a lot easier to live with than under spun.
Some guys like a mag full of lead, I still prefer one round to the head.0 -
TailGunner just about said it all "good job" 0 -
Tailgunner is x-ring on this one.
long bearing surfaces (VLD) on the match bullets, require a faster twist rate to stabilize, and short hunting style bullets with shorter bearing surfaces and standard shaped Ogives don't need as fast a twist to stablize them. As he stated,..a fast twist barrel I.E. 1:9 will stabilize everything whereas a 1:12 barrel would only stabilize the light for caliber pills.
The only problem with fast twist barrels occurs when they are chambered for wildcats or supermagnums that push the envelope (such as the 220swift or 220swiftAI or the RUM's or the 30-378wthby or Lazeroni calibers). These overbore calibers push the bullets so hard and fast,..that a varmint style bullet with a thin jacket and non-bonded core will often reach too high an RPM and the jacket of the bullet will seperate from the core from the extreme rotational forces. that will cause the bullet to either be extremely unstable in it's flight,..or to disintegrate in mid air. This won't happen with the VLD and bonded core bullets due to their bearing surface lengths which will often slow them down as they traverse the rifling lands compared to projectiles of the same weight but of more frangible construction. They also typicaly have a thicker jacket taper.
My rule of thumb,..
30cal 1:10
7mm 1:9
6.5mm 1:8
224cal 1:8 or 1:7
why chase the game when the bullet can get em from here?....
Got Balistics?0 -
"... in what circumstances would one be better over the other?"
What does the twist rate do:
The rate of twist determines the optimum bullet weight (by length) for a given caliber and speed of the bullet by applying the proper spin on the bullet to prevent the bullet form yawing and pitching. Expressed in terms of the number of revolutions per inch of barrel length, this ratio is commonly expressed by designations such as 1:10, 1/10 or 1 in 10 twist, the 1 represents 1 twist, the 10 represents inches of barrel length.
How do you know whats right:
A good rule of thumb is that the heavier and longer a bullet is, the faster the twist rate needs to be to stabilize it in flight, therefore a lighter shorter bullet needs a slower twist rate to give proper bullet spin for correct flight.
Choosing the right rifle:
When choosing a rifle cartridge you should always evaluate what you are going to use the rifle for then select the rifle (or re-barrel) with the correct twist rate for your particular use by bullet weight (length) to gain the most accuracy.
Even manufacturers can goof:
Remington didn't want winchester to run away with the cartridge war with the 270 winchester, as it was well known for its speed and killing power. Remington in (1957) then introduced their speed child cartridge, the 7 millimeter express, at the time remington must have thought they were building a varmint rifle for the simple fact the rifle was introduced with a 1 in 12 twist which allowed it to achieve 3,000 feet per second velocity with 100 grain bullets, but when the shooting public started loading 175 grain bullets for the 7mm express the bullets wouldn't stabilize because the twist rate was to slow, the rifle cartridge got a bad review for being inaccurate and was discontinued. Many years later (1979) remington reintroduced the 7mm express with a faster twist rate and a new name, yes, the so well liked, 280 Remington.
http://www.gunnersden.com/index.htm.rifle-barrel-twist-rates.html
My changes are in red and/or bold.
The classic example to wrong twist also belongs to Remington from 2 years earlier, in 1955 when they released the 244 Rem. with a 1:12 twist thinking that its use would be exclusive to varmint weight bullets. Winchester introduced the .243 Win. in a 1:10 twist which allowed for the use of the longer/heavier 100 gr. bullets which could be used when deer hunting, making the .243 Win. a dual purpose rifle cartridge. The re-introduced 6mm Rem., with a faster twist, never fully recovered.
The circumstances determine what twist you need based on the bullet required to complete the task. Generally, the rifle manufacturers have used the "best" twist for general purposes. It's when we get into areas of very specific uses that we start to look at different twist rates.
Benchrest shooters, using the 6PPC or equivalent, are shooting barrels that have twists of 13 and slower to achieve the minimum RPM with a light bullet for 100, 200 and 300 yard point blank shooting and still get it to the target stabilized. Some Hunter Class benchrest shooters are using the slow 1:15 to 1:18 twist barrels with light 30 cal. bullets for the same reason. Minimum twist for minimum stability to get to the target accurately.
JustC and several others of us that shoot at longer ranges of 500 to 1,000 yards require the faster twists 1:7, 1:8, and 1:9 in order to stabilize the long for caliber VLD bullets and keep them stable past the target. Berger Bullets has taken the step to include the twist rate with each bullet on their website and in their literature. Hornady also does this with their specialty bullets.
Here are a couple of links for twist rates charts:
http://sst.benchrest.com/shilentwist.html
http://www.riflebarrels.com/products/caliber_twist_rates.htm
Twist rate calculators:
http://www.uslink.net/~tom1/twistrate.htm (the best I think)
http://www.z-hat.com/twistrate.htm
http://www.allaboutguns.com/modules.php?op=modload&name=Downloads&file=index&req=viewdownload&cid=6
http://www.again.net/~steve/page8e.htm (lots of resources)
Best.
Corrected spelling errors...0 -
Here is Greenhill's Formula with several examples. You can program any decent spreadsheet or database to make calculations automatically.
Greenhill's was developed 1879, for artillery projectiles, to stabilize their axial rotation.
From the formula you can calculate for any single unknown.
The weight of the projectile is not important. Length is.
Please note I am a muzzleloading rifle shooter. So except for example, data are for ML barrels.
*****
*****
*****A method for estimating correct rate of twist was developed prior to WW I (circa 1879) by Sir George Greenhill. His formula states that the product of the rate of twist and the bullet in calibers should equal 150.
Example:
The optimum rate of twist for Sierra's .30-caliber 200-gr MatchKing would be computed as (t)(l)=150 where:
t=rate of twist
l=bullet length in calibers (1.405"/.308) or 4.561688311688
t=150/4.56
t=32.89 calibers
To convert calibers to inches, multiply by the bullet diameter. In this example, (32.89)(.308)=10.1 inches.
According to Greenhill's formula, then, the proper rate of twist for the Sierra .30-caliber, 200-gr MatchKing is 1:10". That said, and with all due respect to Greenhill, some long-range [smokeless powder] shooters are experimenting with slower rates of twist.
*******
1) .900" long .62-cal (.625") bullet needs 1:65" twist.
2) .900" long .58-cal (.580") bullet needs 1:56" twist.
3) .900" long .54-cal (.540") bullet needs 1:52" twist.
4) .625" RB .62-cal (.625") needs 1:144" twist. AHA!! Weight RB #8776; 366.8 grains. SD = 0.134.
*****
.54-caliber data for Lyman GPR 32" 1:60 twist. Longest bullet .540" Lyman GPR can stabilize is .729".
5) RB (.530") is 223.7 grains, SD = 0.118.
6) Lee R.E.A.L. 300-grain bullet (.540") #90397 (#90398 is double-cavity mold.) is .625" long. SD = 0.147.
7) Lee R.E.A.L. 380-grain bullet (.540") #90399 (#90400 is double-cavity mold.) is .760" long - unsatisfactory. SD = 0.186.
****
.50-caliber data for Lyman GPR 32" 1:60 twist. Longest bullet .500" Lyman GPR can stabilize is .615".
8) RB (.490") is 176.8 grains, SD = 0.105.
9) Lee R.E.A.L. 250-grain bullet (.500") #90393 (#90394 is double-cavity mold.) is .580" long, SD = 0.143.
10) Lee R.E.A.L. 320-grain bullet (.500") #90395 (#90396 is double-cavity mold.) is .730" long - unsatisfactory, SD = 0.183.0 -
quote:Originally posted by Lance
Here is Greenhill's Formula with several examples. You can program any decent spreadsheet or database to make calculations automatically.
Greenhill's was developed 1879, for artillery projectiles, to stabilize their axial rotation.
From the formula you can calculate for any single unknown.
The weight of the projectile is not important. Length is.
A method for estimating correct rate of twist was developed prior to WW I (circa 1879) by Sir George Greenhill. His formula states that the product of the rate of twist and the bullet in calibers should equal 150.
Example:
The optimum rate of twist for Sierra's .30-caliber 200-gr MatchKing would be computed as (t)(l)=150 where:
t=rate of twist
l=bullet length in calibers (1.405"/.308) or 4.561688311688
t=150/4.56
t=32.89 calibers
According to Greenhill's formula, then, the proper rate of twist for the Sierra .30-caliber, 200-gr MatchKing is 1:10". That said, and with all due respect to Greenhill, some long-range [smokeless powder] shooters are experimenting with slower rates of twist.
*******
The problem with giving to much detail when answering a "simple question" is that the majority of the readers can not follow the discussion once it sinks into the "finer points, which is why I gave the simplified answer at the top of this thread.
Greenhill simplified (which you quoted) is a great piece of work, however the "factor" of 150 (the simplified part) used can lead to some problems with calculating the actual twist rate for a given combination. Over approx 2900fps using a factor of 180 gives better results, and at some point below 1900fps a factor of 125 would probably be approperate. IOW, using the same bullet (say a 200SMK), a 30cal carbine and JustC's 300Rum would have to use 2 entirly different rates of twist inorder to obtain the same bullet RPM due to the velocity differences.
The following was copied from Lost Rivers webpage (http://www.lostriverballistic.com/LRB/DocsCorner-Greenhill.cfm)
The Greenhill Formula is a simplified method for determining mathematically the amount of spin necessary to stabilize a bullet. It was worked out in 1879 by Sir Alfred George Greenhill who was a Professor of Mathematics at Woolwich and teaching the Advanced British Artillery Officers Class. It was considered satisfactory for bullets having a density of .392 lbs/cubic inch or greater. (Lead has a density of .409 lbs/ cubic inch, and copper has a density of from .318-.325 lbs/cubic inch, depending on the alloy) The formula is Twist required (in calibers) = 150 divided by the length of the bullet (in calibers). It makes no allowance for nose shape, considering round noses and all spitzers and spire points as the same. It does not work for bullets having a density below .392 lb/ cubic inch. All copper or brass solids and most heavy jacketed bullets have average densities below .392 lbs./cubic inch. Notice I said average, as the formula makes no allowance for bullets of variable construction, linearly. The formula was a shortcut and was useful at the time, as most bullets were roundnoses and were lightly jacketed, if jacketed at all. Because the math is simple, the Greenhill Formula has remained in use to this day. Just a few years ago I had an engineer at a major ammunition manufacturing firm quote me the Greenhill Formula as a method for calculating the spin required to stabilize a long, 10 caliber spitzer, 7mm 175 gr. , variable density, hunting bullet. Needless to say, he was not even close. The Greenhill Formula is accurate when used in the context for which it was intended, but many folks who use it today have forgotten, or never learned that context.
The actual formula is much more complicated It is Gyroscopic Stability (GS) = the spin rate (in radians per second, squared) times the polar moment of inertia, squared, divided by the pitching moment coefficient derivative per sine of the angle of attack times the transverse moment of inertia times the air density times the velocity squared. (My keyboard does not have all the correct symbols and that is why I wrote it out). For the bullet to be stable, GS > 1.0. This is actually a short version as the pitching moment coefficient component is a complicated calculation that derives the center of gravity and the center of reverse air pressure. The equation is basically calculating the linear difference between the center of gravity and the center of reverse air pressure on the nose of the bullet. The greater the difference, the greater the spin required to keep the bullet pointed nose forward. It used to take me about three days to calculate one new design by hand. My computer does it in about 20 seconds, now.
Warren Jensen
Some guys like a mag full of lead, I still prefer one round to the head.0
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