Twist Rates

[nc21]

I have been reading lots on here about twist rates for different bullets working better or worse, Can someone shed some light on twist rate and how it works and how it can effect bullet travel ?

Finially at a time in my life that I can build a gun and think this is something i should know more about !!

Heading to Calgary I hope for some reading tonight

[260 Rem]

Proper twist rate is required to stabilize the bullet so it does not "wobble", which would cause it to go off course and shed accuracy. In exteem cases, the bullet will actually start tumbling. a phenomenon often described as "keyholing" as seen on paper targets. Think of a football thrown as a "spiral" --proper rotation and it flies staight--not enough rotation, and it will "wobble". Same concept with a spinning top.
Google "calculating twist rate for bullets" and you will find a formula that is easy to apply to bullets you shoot.
A 1:8 twist rate means the bore grooves cut into the bullet makes it rotate 1 time for every 8" of travel. With bullets of the same diameter/caliber, longer bullets require a faster rate of twist. For example, a short .30 cal bullet which weighs 115gr can be stabilized by a 1:18 twist whereas its 168 gr cousin will require a 1:13 twist. Not same caliber bullets of the same weight necessarily require the same twist as the calculation factors both diameter and length....so round nose bullets will be different than pointy ones.
When ordering the twist for a build, I calculate the twist rate I will need for the bullets I plan to shoot, and order the slowest twist rate that will cover my needs. My thinking here is based on accuracy potential as many believe that a twist that is too fast causes more torque roll in the bags so to minimize that possbility, I dont want more rotation than is reguired.
Twist rates for the 308 Win are interesting ...as there are some differing opinions. Since most 308 bullets below 170gr will be stabilzed by a 1:13 twist, many builders order a 1:12 to make sure the bullet stabilzes....but if they are going over 175gr bullets for long range...they often opt for twists up to 1:10.
Velocity can also have a somewhat minor impact on the rwist rate required, but would probably be a factor only on the margins of stabilty.
Thanks to gitr's PM, I was able to edit so I didn't embarass myself by saying that the 1:8 twist refers to 1 foot rather than 8". Old misconceptions die hard!

[260 Rem]

Sorry I ran out of EDIT time on the OP...but I was busy chasing mistakes! It is probably noteworthy that although "overstabilization" is not technically possible. Rotating a bullet too fast can cause issues if the bullet is not perfectly concentric. I have also read (not sure if it is fact) that some thin jacketed bullets will actually shed the jacket?

[Pathfinder76]

Twist a barrel fast enough for the heaviest bullet you can use in that particular cartridge and then go a half inch faster. I have never had an issue doing that but have had going the other way.

[Okotokian]

Originally Posted by 260 Rem

Proper twist rate is required to stabilize the bullet so it does not "wobble", which would cause it to go off course and shed accuracy. In exteem cases, the bullet will actually start tumbling. a phenomenon often described as "keyholing" as seen on paper targets. Think of a football thrown as a "spiral" --proper rotation and it flies staight--not enough rotation, and it will "wobble". Same concept with a spinning top.
Google "calculating twist rate for bullets" and you will find a formula that is easy to apply to bullets you shoot.
A 1:8 twist rate means the bore grooves cut into the bullet makes it rotate 1 time for every 8" of travel. With bullets of the same diameter/caliber, longer bullets require a faster rate of twist. For example, a short .30 cal bullet which weighs 115gr can be stabilized by a 1:18 twist whereas its 168 gr cousin will require a 1:13 twist. Not same caliber bullets of the same weight necessarily require the same twist as the calculation factors both diameter and length....so round nose bullets will be different than pointy ones.
When ordering the twist for a build, I calculate the twist rate I will need for the bullets I plan to shoot, and order the slowest twist rate that will cover my needs. My thinking here is based on accuracy potential as many believe that a twist that is too fast causes more torque roll in the bags so to minimize that possbility, I dont want more rotation than is reguired.
Twist rates for the 308 Win are interesting ...as there are some differing opinions. Since most 308 bullets below 170gr will be stabilzed by a 1:13 twist, many builders order a 1:12 to make sure the bullet stabilzes....but if they are going over 175gr bullets for long range...they often opt for twists up to 1:10.
Velocity can also have a somewhat minor impact on the rwist rate required, but would probably be a factor only on the margins of stabilty.
Thanks to gitr's PM, I was able to edit so I didn't embarass myself by saying that the 1:8 twist refers to 1 foot rather than 8". Old misconceptions die hard!

Soooo.... while your description was framed in terms of what twist rate to select if you are doing a build, it also suggest to me that if you know the twist rate for a rifle you already have, you should be able to predict what size bullet is most likely to shoot well from it. Maybe not with 100% certainty, but given your example, if I have a .308 with a 1:12 twist I'm probably less likely to accurately shoot a 180g than a 165g. Do I have that right?

I always buy 150g for my .270 because I want "ooomph" for any game I might hunt... but it might shoot a 140 or 130 of the same bullet better, depending on the twist (and I have no idea what twist my rifle has. Should look it up I guess).

[Dick284]

http://www.ebr-inc.net/articles_Greenhill_Formula.html

Where it all comes from.

[260 Rem]

Originally Posted by Okotokian

Soooo.... while your description was framed in terms of what twist rate to select if you are doing a build, it also suggest to me that if you know the twist rate for a rifle you already have, you should be able to predict what size bullet is most likely to shoot well from it. Maybe not with 100% certainty, but given your example, if I have a .308 with a 1:12 twist I'm probably less likely to accurately shoot a 180g than a 165g. Do I have that right?

I always buy 150g for my .270 because I want "ooomph" for any game I might hunt... but it might shoot a 140 or 130 of the same bullet better, depending on the twist (and I have no idea what twist my rifle has. Should look it up I guess).

Short answer is "no", as trying to make as direct link between accuracy and twist rate is not possible. Knowing the twist rate will only be helpful in being able to calculate which specific bullet may not stabilize. I hope the only link I made between accuracy and twist was my personal preference to go with the minimum twist required to reduce the potential for torque roll..and that is really only a factor when shooting paper off a bench. The 1:12 twist for 308 is IMO, the best for the range of bullets typically shot in 308 Win. Usually, the 180+ gr bullets shot by hunters are not nearly as long as the pointy bullets shot in long range competition..so the 1:12 will stabilize your 180 gr hunting bullet.
I have little experience with the 270, so am not familliar with the twist requirements off the top of my head. I do know the 150 gr game bullets can shoot well in the 270 but I don't think they shine at longer distances from a ballistics perspective -- as I recall, a 150 shot in a 270 is not much different than a 150 from a 308...except the 308 is using 10% less powder.

[elkhunter11]

Soooo.... while your description was framed in terms of what twist rate to select if you are doing a build, it also suggest to me that if you know the twist rate for a rifle you already have, you should be able to predict what size bullet is most likely to shoot well from it. Maybe not with 100% certainty, but given your example, if I have a .308 with a 1:12 twist I'm probably less likely to accurately shoot a 180g than a 165g. Do I have that right?

You don't have it right at all, but the reply by 260Rem is spot on. As for the 150gr bullet supposed having more "ooomph" , that isn't true either. The amount of trauma caused by a bullet, has far more to do with the expansion characteristics, and weight retention of the bullet, than the bullet weight does.

[gitrdun]

Originally Posted by Okotokian

Soooo.... while your description was framed in terms of what twist rate to select if you are doing a build, it also suggest to me that if you know the twist rate for a rifle you already have, you should be able to predict what size bullet is most likely to shoot well from it. Maybe not with 100% certainty, but given your example, if I have a .308 with a 1:12 twist I'm probably less likely to accurately shoot a 180g than a 165g. Do I have that right?

I always buy 150g for my .270 because I want "ooomph" for any game I might hunt... but it might shoot a 140 or 130 of the same bullet better, depending on the twist (and I have no idea what twist my rifle has. Should look it up I guess).

You can easily figure it out yourself using a cleaning rod with a swiveling handle and a bore brush. Make a mark on the handle with a marker and a corresponding mark on the rod. Draw the rod through the bore for a full revolution (using the 2 marks as reference). Measure the lenght that the rod was drawn through the bore for a full revolution. Voila, your twist rate.

[Pathfinder76]

Or look up the manufacturers specs online.

[Okotokian]

Originally Posted by elkhunter11

As for the 150gr bullet supposed having more "ooomph" , that isn't true either. The amount of trauma caused by a bullet, has far more to do with the expansion characteristics, and weight retention of the bullet, than the bullet weight does.

I get the apples/oranges comparison if you are comparing two completely different bullet types. I was speaking/thinking of my usual 150g Winchester XP3 round versus a 130g XP3 round. The former is larger, has almost identical velocity and over 200 more ft-lbs of energy at any range you choose to select than the 130g. How could it not have more "oomph"?

Heck, even in trajectory the 130 is listed as only being a single inch flatter at 500 yards. I actually can't for the life of me see why I would select the 130 to use. But then, I miss many things. LOL

And thanks guys for the twist measurement suggestions.

[elkhunter11]

Originally Posted by Okotokian

I get the apples/oranges comparison if you are comparing two completely different bullet types. I was speaking/thinking of my usual 150g Winchester XP3 round versus a 130g XP3 round. The former is larger, has almost identical velocity and over 200 more ft-lbs of energy at any range you choose to select than the 130g. How could it not have more "oomph"?

Heck, even in trajectory the 130 is listed as only being a single inch flatter at 500 yards. I actually can't for the life of me see why I would select the 130 to use. But then, I miss many things. LOL

And thanks guys for the twist measurement suggestions.

The real question is,what is oomph, and how do you measure it? The only thing that concerns me, is the resulting wound channel, because that is what causes death.

[ShawnM]

http://www.chuckhawks.com/rifling_twist_rate.htm

This is a good read for basic information on twist rates for rifles.

[super7mag]

Originally Posted by chuck

Twist a barrel fast enough for the heaviest bullet you can use in that particular cartridge and then go a half inch faster. I have never had an issue doing that but have had going the other way.

That's what Charlie Sisk recommends , makes sense to me.