- Location
- Grande Prairie, Alberta
can someone explain to me barrel twist and rifling? right down to basics. what makes a difference, what's better when etc? thanks.
Barrel Twist
- Thread starter FlyingHigh
- Start date
spins the bullet to stabilize it during flight. Basic enough?
Try throwing a football with no spin and see if it travels nice and straight. Pretty much the same dynamics to get a bullet to fly nice and straight with no yaw. The rifling in your barrel introduces spin to the bullet as Peyton Manning introduces spin to a football, all to get the projectile to get it where you want.
- Location
- Somewhere on the Hudson Bay Coast
A bullet needs to spin around a linear axis to be stable in flight. The rate of twist is expressed as 1 full turn to X number of inches or in the case of a 1 in 10 twist, it would be expressed as 1:10. This rate of twist in a rifle barrel determines the rate of spin induced to the bullet. To determine the actual spin induced to the bullet you multiply the velocity by the rate of twist and divide by 12. In the case of a bullet which esxits a 1:10 barrel at 2700 fps the calculation is:
2700X10/12 = 2250 ### (revolutions per second)
The length of the bullet determines how fast this spin must be. If we have two bullets of the same caliber and weight, the one with the flat base, flat nose and parallel sides will be shorter than the bullet that has a long sharp nose, and a boat tail. Conversely if we have two bullets of the same caliber and shape but different weights, the heavier bullet will be longer. The shorter bullet will stabilize with less spin than the longer bullet. The Greenhill formula is a simple way to determine the rate of twist you need to stabilize a given bullet.
Twist = 150 X D2/L (150Xdiameter squared/length)
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
Roughly 30% of the energy created by the propellant gases is needed to impart spin on the bullet. If we did not need to spin our bullets we could achieve much higher velocity with the same pressure. Therefore, generally speaking, the faster the rate of twist, the greater the detriment to velocity. If you have two 30 caliber barrels of equal length, one with a 1:12 twist and the other with a 1:8 twist, if identical loads are fired from both barrels, all things being equal, the 1:12 barrel will produce higher velocity than the 1:8 barrel.
Any given twist will stabilize a broad range of bullet weights (lengths). Consider two 30/06 rifles. The 1:10 barrel will stabilize all 30 caliber bullet weights from 100 up to 220 grs. A 1:12 barrel will stabilize all the light bullets, but shoot a 200+ gr match bullet and that the bullet will yaw badly as it hits the target. A 1:8 twist barrel is needed to stabilize the very long 240 gr Match Kings, but the 1:10 barrel will stabilize the shorter 240 gr Woodleigh big game bullet.
Speaking of game, the rate of twist can dramatically effect the bullets performance on game. Those who shoot varmint rifles, particularly in populated areas need bullets that will break up when they hit the ground without ricocheting. A bullet from a fast twist barrel will break apart much more quickly than from a slow twist barrel. I found that Sierra Blitz and Hornady SX bullets would often not even make it to the target when fired from my 1:7 twist barrel. Naturally varmint sized game is badly torn up when such bullets connect, and while the kill is humane, pelts would be badly damaged. On big game a fast twist has a different effect. The amount of penetration of a game bullet is effected by a phenomenon known precession. Precession is the amount of torque imposed on the bulllet when it hits a game animal. The faster the spin of the bullet the less effect this torque has, so the deeper the penetration.
As you can see there is no best twist. You must decide what your rifle/bullet combination is for and choose the appropriate bullet and twist accordingly. It can either be complicated or simple. Target shooters prefer to have the precise twist for their favorite bullet weight, but the rest of us can get by with a twist that will stabilize a broad range of bullet weights.
2700X10/12 = 2250 ### (revolutions per second)
The length of the bullet determines how fast this spin must be. If we have two bullets of the same caliber and weight, the one with the flat base, flat nose and parallel sides will be shorter than the bullet that has a long sharp nose, and a boat tail. Conversely if we have two bullets of the same caliber and shape but different weights, the heavier bullet will be longer. The shorter bullet will stabilize with less spin than the longer bullet. The Greenhill formula is a simple way to determine the rate of twist you need to stabilize a given bullet.
Twist = 150 X D2/L (150Xdiameter squared/length)
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
Roughly 30% of the energy created by the propellant gases is needed to impart spin on the bullet. If we did not need to spin our bullets we could achieve much higher velocity with the same pressure. Therefore, generally speaking, the faster the rate of twist, the greater the detriment to velocity. If you have two 30 caliber barrels of equal length, one with a 1:12 twist and the other with a 1:8 twist, if identical loads are fired from both barrels, all things being equal, the 1:12 barrel will produce higher velocity than the 1:8 barrel.
Any given twist will stabilize a broad range of bullet weights (lengths). Consider two 30/06 rifles. The 1:10 barrel will stabilize all 30 caliber bullet weights from 100 up to 220 grs. A 1:12 barrel will stabilize all the light bullets, but shoot a 200+ gr match bullet and that the bullet will yaw badly as it hits the target. A 1:8 twist barrel is needed to stabilize the very long 240 gr Match Kings, but the 1:10 barrel will stabilize the shorter 240 gr Woodleigh big game bullet.
Speaking of game, the rate of twist can dramatically effect the bullets performance on game. Those who shoot varmint rifles, particularly in populated areas need bullets that will break up when they hit the ground without ricocheting. A bullet from a fast twist barrel will break apart much more quickly than from a slow twist barrel. I found that Sierra Blitz and Hornady SX bullets would often not even make it to the target when fired from my 1:7 twist barrel. Naturally varmint sized game is badly torn up when such bullets connect, and while the kill is humane, pelts would be badly damaged. On big game a fast twist has a different effect. The amount of penetration of a game bullet is effected by a phenomenon known precession. Precession is the amount of torque imposed on the bulllet when it hits a game animal. The faster the spin of the bullet the less effect this torque has, so the deeper the penetration.
As you can see there is no best twist. You must decide what your rifle/bullet combination is for and choose the appropriate bullet and twist accordingly. It can either be complicated or simple. Target shooters prefer to have the precise twist for their favorite bullet weight, but the rest of us can get by with a twist that will stabilize a broad range of bullet weights.
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In some calibers it does not make much difference, unless you intend to shoot an unusual bullet.
If bullet spin is inadequate the bullet will tumble and accuracy is nil. If the bullet is spun too fast, groups will be slightly bigger and in some cases, the bulets can blow up in mid air.
Long range shooting is best with long pointy (and heavy) bullets. I use Sierra 80 gr bullets in my 223 and 22-250 Ackely. They need a 1-9 or faster barrel. If I was only going to shoot benchrest at 100-200 yards, I would want to shoot lighter custom made bullets with flat bases. A 1:12 or 1:14 barrel would be best.
My 308 Win target barrels are all 1:14 or 1:13 beacause I don't shoot anything heavier than a 175. My hunting barrels in that caliber are all 1:12 or 1:10 because I prefer heavy bullets for hunting.
Was your question for general information or do you have a caliber and application in mind?
If bullet spin is inadequate the bullet will tumble and accuracy is nil. If the bullet is spun too fast, groups will be slightly bigger and in some cases, the bulets can blow up in mid air.
Long range shooting is best with long pointy (and heavy) bullets. I use Sierra 80 gr bullets in my 223 and 22-250 Ackely. They need a 1-9 or faster barrel. If I was only going to shoot benchrest at 100-200 yards, I would want to shoot lighter custom made bullets with flat bases. A 1:12 or 1:14 barrel would be best.
My 308 Win target barrels are all 1:14 or 1:13 beacause I don't shoot anything heavier than a 175. My hunting barrels in that caliber are all 1:12 or 1:10 because I prefer heavy bullets for hunting.
Was your question for general information or do you have a caliber and application in mind?
- Location
- Grande Prairie, Alberta
thanks Boomer and Ganderite! that was exactly what i was looking for. 
a little bit of both, actually. i'm kicking around the idea of building up a precision "sniper" rifle that is extremely accurate, but light enough to pack around. i'm pretty sure it would be a .308.i was wondering about barrel twist because i'd want to know what to look for in an accurate gun. i'm also wondering if i would be buying an aftermarket barrel. like i said, it's still just an idea in progress, and i don't even know what gun i want to start off with, or what loads i'd be shooting.
a little bit of both, actually. i'm kicking around the idea of building up a precision "sniper" rifle that is extremely accurate, but light enough to pack around. i'm pretty sure it would be a .308.i was wondering about barrel twist because i'd want to know what to look for in an accurate gun. i'm also wondering if i would be buying an aftermarket barrel. like i said, it's still just an idea in progress, and i don't even know what gun i want to start off with, or what loads i'd be shooting.
Boomer brought it up, but optimal twist rates are based on the bullet length, not its weight, however there is a proportional relationship between bullets that weigh more and those that are longer.
Look-up the "Greenhill formula" on the net and it will tell you what you need to know about twist rates.
You need to decide what sort of shooting you are going to do and the distances you want optimum results for.
in .308, A 1:12" twist will stabilize almost all 155-180 grain VLD bullets and is a good middle of the road choice. 1:10 is a very popular choice for long range shooters that will use heavy high ballistic coefficient bullets and I personally sell more 1:10 30 cal barrels than all others combined.
The numbers of grooves make no difference to accuracy whatsoever and neither do the shape of the lands. Their are many compelling anecdotes that endorse every style of groove/land style. Your gunsmith however may not like working with barrels that have odd numbered grooves because it is extremely difficult to accurately measure the bore diameter... talk with him/her.
Look-up the "Greenhill formula" on the net and it will tell you what you need to know about twist rates.
You need to decide what sort of shooting you are going to do and the distances you want optimum results for.
in .308, A 1:12" twist will stabilize almost all 155-180 grain VLD bullets and is a good middle of the road choice. 1:10 is a very popular choice for long range shooters that will use heavy high ballistic coefficient bullets and I personally sell more 1:10 30 cal barrels than all others combined.
The numbers of grooves make no difference to accuracy whatsoever and neither do the shape of the lands. Their are many compelling anecdotes that endorse every style of groove/land style. Your gunsmith however may not like working with barrels that have odd numbered grooves because it is extremely difficult to accurately measure the bore diameter... talk with him/her.
- Location
- Grande Prairie, Alberta
thanks for the info Obtunded. i don't have any gunsmiths locally here. i'll have to track one down. what twist would you suggest for accurate shooting out to say 600 - 800 yards and occasionally 1000? would i have to have a barrel custom made, or would i be able to find a standard production barrel? (again, not sure on what gun i want yet)
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In 308? 1:10, 1:11, 1:12. Personally I prefer the 1:10, but that's because I like to use longer/heavier bullets. FWIW - dan
Boomer and Obtunded have given you excellent info. and advice but if i may, as small correction. They are right it's length not weight that matters BUT, it is NOT the length of the bullet BUT length of the bearing surface of the bullet in the barrell.
According to Hatchers handbook and a few other sources i've read.
The more bearing surface that the bullets have in the barrell/rifling the more twist they need.
Take two bullets of equal weight but different shapes , one has a long ogive one has a short ogive, the shorter one should have more bearing surface area as both are the same weight (same amount of material) and it will need more spin to stabalize it.
my 2 cents.
M.
The bullets i've been designing have a very long Ogive and need less spin. I'm about to shorten the ogive and try to get more bearing surface to see how this effects things at longer yardages.
According to Hatchers handbook and a few other sources i've read.
The more bearing surface that the bullets have in the barrell/rifling the more twist they need.
Take two bullets of equal weight but different shapes , one has a long ogive one has a short ogive, the shorter one should have more bearing surface area as both are the same weight (same amount of material) and it will need more spin to stabalize it.
my 2 cents.
M.
The bullets i've been designing have a very long Ogive and need less spin. I'm about to shorten the ogive and try to get more bearing surface to see how this effects things at longer yardages.
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- Location
- Somewhere on the Hudson Bay Coast
Ice pic, I disagree. A heavy for caliber VLD has a very short bearing surface and requires a very fast twist to stabilize that bullet. A flat based, flat nosed bullet of equal weight with parallel sides and a long bearing surface will stabilize with less twist, such as my comparison between the 240 MK and the 240 gr Woodleigh soft point. Another comparison is the 105 gr Speer and the 107 gr MK, both in .243, again the MK needs a faster twist then the game bullet with the long bearing surface.
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Bearing surface has no real effect on gyroscopic stabilty but bullet shape does. A bullet of the same length but with a longer bearing surface will carry more of it's weight on the periphery of the bullet and the gyroscopic effect will be greater. Meaningless in the real world but there it is.
If I was building a 308 for ranges over 300 meters, I would always use a 12 twist. It is fast enough to stabilize any bullet I would be likely to shoot in the 308 (up to 200 match kings) and probably ideal for some of the longer 155's. Regards, Bill.
If I was building a 308 for ranges over 300 meters, I would always use a 12 twist. It is fast enough to stabilize any bullet I would be likely to shoot in the 308 (up to 200 match kings) and probably ideal for some of the longer 155's. Regards, Bill.
Well, I understand what your trying to say,,,,,, but,,,,, and i'll have to try and look it up again, I lent the book to someone. I was getting my info from a group of guys that did these tests for a living in the 40's, 50's and 60's and had names that started with "Dr."
Since I didn't have access to the same test equipment they had i was willing to take their word for it.
M.
BTW Leeper, your argueing against your self, first you say "bearing surface has no real effect on gyroscopic stability" then you say
"with a longer bearing surface will carry more of its weight on the perphery of the bullet and the gyroscopic efect will be greater."
Since I didn't have access to the same test equipment they had i was willing to take their word for it.
M.
BTW Leeper, your argueing against your self, first you say "bearing surface has no real effect on gyroscopic stability" then you say
"with a longer bearing surface will carry more of its weight on the perphery of the bullet and the gyroscopic efect will be greater."
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- Location
- Somewhere on the Hudson Bay Coast
Ice-Pick, the bullet with the longer bearing surface carries a greater portion of its weight away from it's dimensional center than does the VLD. Once there is sufficient spin to stabilize the bullet, it is no less stable than the stabilized VLD. If this is a contradiction, then because a .510" bullet carries a greater mass away from it's dimensional center than does the .224, the .224 must be more stable. Certainly this does not stand up to real world scrutiny provided that the bullet's dimensional center matches its center of gravity, and that the rate of spin is sufficient to produce stability.
