twist rate

[deserthntr]

just curious about twist rate in barrels, i know what it is and the 1:9 of 1:12 means. but is more twist always better? can there be too much twist for a barrel and it affect the bullet where its throwing sideways? i guess i would just kinda like insight to barrel twist rates, whats good what isnt.

[firstcoueswas80]

just curious about twist rate in barrels, i know what it is and the 1:9 of 1:12 means. but is more twist always better? can there be too much twist for a barrel and it affect the bullet where its throwing sideways? i guess i would just kinda like insight to barrel twist rates, whats good what isnt.

 

 

Yes, twist should be matched to bullets. For example: a heavier bullet shoots slower so it needs a FASTER twist to get the revolutions going to stabalize. A lighter, faster bullet requires a slower twist because of the higher velocity it gets the revolutions going... Take Berger VLD bullets, they normally reccomend a fast-for-cartridge twist because their bullets are normally bigger and heavier than the everyday bullet.

[deserthntr]

ok cool. so say i am shooting a 185 grain bullet, i need a say 1:12 twist. but then in the same caliber i start shooting a 140, i need maybe a 1:9? and the smaller the caliber would i go the slower the twist?

[slopoke36]

Correct me if I am wrong but for the heavier bullet you would need the 1:9 and lighter bullet would take the 1:12.

 

They way I understood it was 1 revolution per 12 inches or 9 inches.

[willhunt4coues]

I was told I needed a 165 or 168 in my 20" 1 in 12 twist .308. That was a gunsmith and a gun store that told me this. My gun shoots great with this ammo and even if someone told me to shoot 150 I wouldn't because they actually drop more at further distances.

[firstcoueswas80]

Slopoke is correct

[Siwash]

This link has some basic insight: http://www.shilen.com/calibersAndTwists.html

[jeffro]

I was told I needed a 165 or 168 in my 20" 1 in 12 twist .308. That was a gunsmith and a gun store that told me this. My gun shoots great with this ammo and even if someone told me to shoot 150 I wouldn't because they actually drop more at further distances.

 

Your 1-12 T will shoot anything from 125gr to 208gr. With the 208s you may need to get your speed up 2600fps for the bulets to stableize.

[308Nut]

A few things to concider about twist rates.

 

1: It is bullet length that has the most affect on stability. In most cases, heavy bullets require more twist than light bullets but this is simply due to their length. To illustrate, an all copper bullet (200 grain/30 cal) requires substantially more twist than a jacketed lead 30 cal 200 grain bullet. This is because the specific gravity of copper is lower than jacketed lead. For the bullet to be 200 grains, it has to be longer. It is this length/weight ratio that is the major concideration when selecting the proper twist.

 

2: Speed comes into play a bit, but not as much as you might think. For example:

 

Running the 208 grain AMAX in an 11 twist barrel at 2600 FPS yields a gyroscopic stability factor of 1.28. Bump it up to 3600 FPS and it becomes 1.43. By moving to the 12 twist it becomes 1.08 and 1.2 respectively. Going to the 10x they become 1.55 and 1.73. So, speed does come into play but you cannot substitute speed for twist. Raw RPMs by themselves do not cut it. 1.08 is getting close to unstable. 1.0 and above is concidered stable. If you have a load that has a stability factor of 1.05 at 5000' in 80 degree air, you can bet that at 1000' in 30 degree air that the SG will be below 1.0 and unstable. 1.3 to 1.5 is generally accepted as optimum. This way they are not way overspun and they are not at risk of becoming unstable in a changing enviornment.

 

3: Excessive spinning for longrange shooting may or may not hurt you but it will not help you. The exception to that rule is in SOME circumstances, excessive twist can help a bullet stay stable after it hits the transonic wall. There are many factors that come into play for a bullet surviving the transonic zone and it is VERY unpredictable. That said, bullets tend to increase stability as they travel until they go subsonic. There is no need to spin them to oblivion to ensure they reach the target stable. They either are stable or they are not. This is not to be confused with what some refer to as the yaw of response. In other words, they may wobble a bit when they exit the muzzle and 'settle down' later. This is not instability. Ballistic coefficients can be negatively affected by too much spinning as can running them on the ragged edge on the low side. When spun properly, the BC will be at its best albiet we are talking a VERY small amount. Not even enough to make it a big concideration unless 2000 yards is the objective. Using a twist that is tighter than needed is less of a problem from an accuracy standpoint when mono metal bullets are used and it is more critical when jacketed bullets are used. If there are any jacket concentricity issues, accuracy can suffer and the faster you spin them, the worse it gets. With mono metal bullets, there are no concentricity isses. The next negative is the amount of spin drift that occurs. You cannot eliminate spin drift but you can minimize it. One of the easiest ways to minimize it is to not spin your bullets any more than you need to.

 

Even with all of that in mind, I dont lose alot of sleep over twist rates. Get a twist for the longest bullet you will be shooting at the velocity you will be using and be happy. If you want to shoot shorter bullets, so be it. They may not be perfectly optimum but optimum enough. If we were talking 1000 yard bench rest rifles here, it would be different. For hunting rifles, even longrange hunting rifles, it isnt nearly as big a deal.

 

Most ballistic software has twist rate calculators. I also believe JBM online ballistics has a twist rate calculator. They take into account the bullet's material, length, weight, overall design, caliber, altitude, temp, humidity, etc.......and will generate the best twist value. Below is a link to Lilja's web page that has available for download a twist calculator. It is quite cumbersome but if used properly can get you pretty close.

 

Shoot straight!

[.270]

thanks 308nutjob. i read that whole thing and now my head hurts. owww. Lark.

[willhunt4coues]

I was told I needed a 165 or 168 in my 20" 1 in 12 twist .308. That was a gunsmith and a gun store that told me this. My gun shoots great with this ammo and even if someone told me to shoot 150 I wouldn't because they actually drop more at further distances.

 

Your 1-12 T will shoot anything from 125gr to 208gr. With the 208s you may need to get your speed up 2600fps for the bulets to stableize.

 

Cool thanks for the info. I am shooting the Federal Gold Medal Match 168 hollow points. Has anyone taken an animal with it? Should I use the Federal Vital Shock 165? They have the same drop till you get out to 800 900 & 1000 they are a little different but I can compinnsated if I decide to go that far.

[firstcoueswas80]

Clay, whats the last word in the bullet description..... MATCH. I would NOT use them for hunting.

[daryl_s]

A few things to concider about twist rates.

 

1: It is bullet length that has the most affect on stability. In most cases, heavy bullets require more twist than light bullets but this is simply due to their length. To illustrate, an all copper bullet (200 grain/30 cal) requires substantially more twist than a jacketed lead 30 cal 200 grain bullet. This is because the specific gravity of copper is lower than jacketed lead. For the bullet to be 200 grains, it has to be longer. It is this length/weight ratio that is the major concideration when selecting the proper twist.

 

2: Speed comes into play a bit, but not as much as you might think. For example:

 

Running the 208 grain AMAX in an 11 twist barrel at 2600 FPS yields a gyroscopic stability factor of 1.28. Bump it up to 3600 FPS and it becomes 1.43. By moving to the 12 twist it becomes 1.08 and 1.2 respectively. Going to the 10x they become 1.55 and 1.73. So, speed does come into play but you cannot substitute speed for twist. Raw RPMs by themselves do not cut it. 1.08 is getting close to unstable. 1.0 and above is concidered stable. If you have a load that has a stability factor of 1.05 at 5000' in 80 degree air, you can bet that at 1000' in 30 degree air that the SG will be below 1.0 and unstable. 1.3 to 1.5 is generally accepted as optimum. This way they are not way overspun and they are not at risk of becoming unstable in a changing enviornment.

 

3: Excessive spinning for longrange shooting may or may not hurt you but it will not help you. The exception to that rule is in SOME circumstances, excessive twist can help a bullet stay stable after it hits the transonic wall. There are many factors that come into play for a bullet surviving the transonic zone and it is VERY unpredictable. That said, bullets tend to increase stability as they travel until they go subsonic. There is no need to spin them to oblivion to ensure they reach the target stable. They either are stable or they are not. This is not to be confused with what some refer to as the yaw of response. In other words, they may wobble a bit when they exit the muzzle and 'settle down' later. This is not instability. Ballistic coefficients can be negatively affected by too much spinning as can running them on the ragged edge on the low side. When spun properly, the BC will be at its best albiet we are talking a VERY small amount. Not even enough to make it a big concideration unless 2000 yards is the objective. Using a twist that is tighter than needed is less of a problem from an accuracy standpoint when mono metal bullets are used and it is more critical when jacketed bullets are used. If there are any jacket concentricity issues, accuracy can suffer and the faster you spin them, the worse it gets. With mono metal bullets, there are no concentricity isses. The next negative is the amount of spin drift that occurs. You cannot eliminate spin drift but you can minimize it. One of the easiest ways to minimize it is to not spin your bullets any more than you need to.

 

Even with all of that in mind, I dont lose alot of sleep over twist rates. Get a twist for the longest bullet you will be shooting at the velocity you will be using and be happy. If you want to shoot shorter bullets, so be it. They may not be perfectly optimum but optimum enough. If we were talking 1000 yard bench rest rifles here, it would be different. For hunting rifles, even longrange hunting rifles, it isnt nearly as big a deal.

 

Most ballistic software has twist rate calculators. I also believe JBM online ballistics has a twist rate calculator. They take into account the bullet's material, length, weight, overall design, caliber, altitude, temp, humidity, etc.......and will generate the best twist value. Below is a link to Lilja's web page that has available for download a twist calculator. It is quite cumbersome but if used properly can get you pretty close.

 

Shoot straight!

 

When my dad was trying out some different bullet types and weights he had one particular bullet that had tighter groups at 300 than it did at 100 yards. Could this be an example of what you are talking about here??

 

Good info by the way. Thanks for sharing. And +1 on what Lark said, my head hurts a little

[deserthntr]

Clay, whats the last word in the bullet description..... MATCH. I would NOT use them for hunting.

 

 

but isnt that what berger's hunting bullets are? if it has a hollow point it will still open up, right?

[308Nut]

I am shooting the Federal Gold Medal Match 168 hollow points. Has anyone taken an animal with it? Should I use the Federal Vital Shock 165? They have the same drop till you get out to 800 900 & 1000 they are a little different but I can compinnsated if I decide to go that far.

 

I have taken several animals with the 168 SMK and the 190 SMK and at close range (250 yards or less) they work fair. After that they pencil hole right through. I personaly dont use SMK's anymore for hunting. Hornady AMAX are 'match bullets' that work awesome on deer size game out at longer ranges. They will however explode when shot at magnum velocities or when heavy bone is hit so you still have to be very carefull and selective.

 

The accubonds in my opinion are about the best balance between accuracy and terminal performance.

 

There are no magic bullets. AMAXs explode up close but open nice at very long range, accubonds open up nice at close and medium ranges and pencil at very long range. Choose wisely.