Written by Sierra Bullets Ballistic Technician Paul Box
Based on the questions we get on a daily basis on our 800 line, twist is one of the most misunderstood subjects in the gun field. So let’s look deeper into this mystery and get a better understanding of what twist really means.
When you see the term 1-14″ or 1-9″ twist, just exactly what does this mean ? A rifle having a 1-14″ twist means the bullet will rotate one complete revolution every fourteen inches of the barrel. Naturally a 1-9″ turns one time every nine inches that it travels down the barrel. Now, here’s something that some people have trouble with. I’ve had calls from shooters thinking that a 1-14″ twist was faster than a 1-9″ because the number was higher with the 1-14″. The easiest way to remember this is the higher the number, the slower the twist rate is.
Now, the biggest misconception is that if a shooter has a .223 with a 1-8″ twist, his rifle won’t stabilize a 55 gr. bullet or anything lighter. So let’s look at what is required. The longer a bullet is for its’ diameter, the faster the twist has to be to stabilize it. In the case of the .223 with a 1-8″ twist, this was designed to stabilize 80 gr. bullets in this diameter. In truth the opposite is true. A 1-8″ will spin a 55 gr. faster than what is required in order to stabilize that length of bullet. If you have a bullet with good concentricity in its jacket, over spinning it will not hurt its’ accuracy potential.
Think of it like tires on your truck. If you have a new set of tires put on your truck, and they balance them proper at the tire shop, you can drive down a street in town at 35 MPH and they spin perfect. You can get out on the highway and drive 65 MPH and they still spin perfect. A bullet acts the same way.
Once I loaded some 35 gr. HP bullets in a 22-250 Ackley with a 1-8″ twist. After putting three shots down range, the average velocity was 4584 FPS with an RPM level of 412,560. The group measured .750″ at 100 yds. This is a clear example that it is hard to over stabilize a good bullet.
Sierra Bullets 
Here’s a couple questions: 1. Does over-stabilization effect the bullet’s performance as it passes from super-sonic to sub-sonic? 2. Is there any advantage to gain-twist rifling when trying to use a fast twist rate? Thanks!
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Actually there is some thought that it does. I’m not sure if it’s actually been proven. Gain twist have come and gone thru the years and I will admit to not knowing anything about the effects of that. Sorry.
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A lot more to this than this. Issues of transonic transition and the Mach 1.3 factor, for those issue you need to look up super sonic flight on you tube , bullets react in similar fashion. Not all stabilized bullets go through the Mach 1.3 stabilized and yet they are some of the best 300 meter bullets ever produced, others don’t even notice the difference when passing to sub-sonic velocities. In a recent court case the defense tried to throw out evidence of damaged wood shot at 1 foot on the idea that the bullet hadn’t gone to sleep , want to comment on that one , please do ..
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This subject was dealing with everyday question we get concerning twist rates and what weight (length) bullets would stabilize. Naturally when you get to the point of a bullet going subsonic we are dealing with something more complex. However this is some thought on the effects of the extra RPMS a bullet has and the effects it has when going thru the transonic range. As to your court case, that was lawyers, not ballisticians.
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My partner has Tikka T3 Lite (normal weight barrel) with8″ twist. With #1365 55gn SBT it has stunning accuracy .20MOA with 26gn of Varget! A friend (who shoots world class Bench Rest), has a Tikka T3 Varmint (heavy barrel) 8″ twist. With the same 55gn #$1365, it becomes a shot gun and wants nothing less than 75gn!
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The T3 Super Lite (.223) is actually a Sako barrel – the same one that’s on the Sako Finlight, whereas the T3 Varmint barrel is just made on the same equipment (as I’m lead to believe) for the Tikka. The Super Lite is a very nice barrel. A friend of mine has one, and it shoots 35gr pills .25MOA. My Sako Varmint shoots the 1365 GameKing at .3MOA with a mv of 3740fps (22-250 with 37.4gr AR2208, which is marketed in the US as Varget – I’m in Aus), but that’s only my 2nd load for this rifle so I hope to improve it over time. I’m keen to see any evidence of negative effects of over stabilisation, apart from hyper volatility of the bullet.
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Some barrels are just this way, but it doesn’t have anything to do with the twist.
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Does having a higher twist rate barrel (meaning more rotations per inch, 1-7″) require lower velocities due to more rifling per inch?
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No, they do not.
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I would have expected velocities to have been reduced a tad by faster rifling, but I haven’t found this to be the case. I do, however, expect the lands to wear faster in a faster twist, as more rotational force is applied to the bullet as it engraves. Any thoughts on these two issues? I’d also expect a bigger jump (seating further out) to result in more wear on the lands, as the bullet would be travelling faster as it engages, and hence would encounter a greater rotational force applied by the lands as it engraves – more linear speed = more rotational speed. Thoughts??
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We haven’t noticed any difference in throat wear between a slower twist vs a fas
\ter one.
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Mr Box concluded perfectly his comments on stabilization with a shattering of the term “over-stabilized.” It is a pet peeve to hear or read such reference.
For those in disagreement answer the following to help grasp the non-existence of overstabilization: does a fifth leg to a folding card table add stability, a gyroscope standing motionless needs how many ore RPMs to stand even more motionless, does a train rolling on straight level tracks need a 3rd rail to add stability?
The obvious answer is no. The better term for what is referenced is “excessive rotational velocity.” Hence we link the reality of a necessary minimum RV to achieve stability with the realm and notion of achieving too much of it.
Just sayin’!!
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If there is no such thing as spinning a bullet too fast, why don’t you shoot 1:1 twist barrels?
The fact is – there is an optimal RPM (i.e., RV) range for rotational stability of an object in motion through air. Too fast or too slow is suboptimal. Over rotation does cause an upward pitch to the bullet, which increases air friction (essentially lowering effective BC). Over rotation can also magnify imperfections in a bullets’ construction / concentricity during flight.
The DoD spent a lot of money and fired millions of rounds to ammo when switching from M193 fired in 1:12″ barrels to M855 fired in 1:7″ barrels. The evidence was conclusive and widely reported (even in American Rifleman) that the accuracy of M193 fired in 1:7″ twist barrels was significantly worse than from 1:12″. Granted, these aren’t $1 each match bullets… but it clearly represents that spinning an “average bullet” faster than optimal produces suboptimal results.
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I have found that “overstabilization” limits long range dope of the lighter bullets. 2 barrels aiming at the same angle for a normal 600 yard line zero. both loaded with the same 55 grain bullet. One is a 12 twist and one is a 6.5 twist. The 12 twist hits the 10 ring and the 6.5 twist is in the dirt low. Why? I think the bullet reaches the top of the arc and starts downward point first like normal. Like a good football spiral, tip first the whole way.The 6.5 twist bullet keeps it’s nose up like a speedboat from gyroscopic spin that can’t be overcome and comes down belly first like a speedboat on water. The bullet is like a gyro top and keeps its angle true through the whole flight and loses it’s BC and aerodynamics after peak height of trajectory.
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It’s more likely (to my thinking) the 6.5″ twist is resulting in a lower MV. Have you chrony’d the two to verify the same MV? As soon as a bullet begins to deviate from it original axis, it will wobble and wander
. This is under stabilised.
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I have a question I thought about.
if I have 2 different .223 rifles, first one is 1:8 twist 16″ barrel. Second is 1:12 twist 26″ barrel.
What if I fired a heavy for caliber bullet like a 68gr or 77gr, something that isn’t supposed to stabilize in a 1:12 twist.
The 1:8 twist rifle would have the bullet doing 2 full revolutions (1 turn in 8 inches, 16 inch barrel)
The 1:12 twist rifle would have the bullet doing 2 full revolutions + an additional 1/8th a revolution, so my question is, wouldn’t this mean that this 1:12 twist rifle with a long 26″ varmint barrel be able to stabilize a heavy for caliber bullet?
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The rate of twist remains unaltered regardless of barrel length, ie. 1:8 or 1:12, it is merely in contact for a longer period of travel with the longer barrel. The stability can be verified by using a manufacturer ‘s information or the Greenhill stability factor calculator.
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I had a talk with the bullet experts, “Sierra” and asked about twist rates vs velocity for stabilization on a 6.5 caliber bullet. The answer I got was the velocity increase wouldn’t help make it more stable without the minimum twist required. Now skip to the 224 Valkyrie on the 90gr.if the velocity is greater than 2,650fps you can use a 1/7 instead of 1/6.5. Well there goes the neighborhood. This must be the only bullet designed for that. You have my email for response, just thought I would throw it out for others with questions.
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You’re not diving deep enough into this. At 100 or 200 yards ok but…stretch that scenario out to 600-1000 yards… A 70 grain bullet in that scenario would travel point first through its complete arc and have predicable dope, whereas the 35 grain bullet,once it hit the top of its arc, would continue to be nose up like a speed boat on water, traveling belly first and nose up out to the target. It would not be correctly correlating with the proper dope for elevation or windage as it would with the (proper) lesser twist it requires. The bullet would act more like a gyro than a football in flight. The aerodynamics wouldn’t be optimal due to the bullets attitude in flight. Shedding velocity at a higher rate than normal and falling victim to wind drift at an abnormal rate as well.
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Spin drift. Or Coriolis Effect. I am a little surprised this is nearly unmentioned, as Mr. Robert has, as a matter of discussion. I think there is a community whom understands rotational spin drift and buys into the fact that increasing/decreasing the projectile rotational spin, beyond it’s optimum spin rate, also assumes an increase/decrease in the the percentage of spin drift (same direction as rifling) therefore less accurate or predictable. This would certainly seem to be an expedential effect as distance increases and less effect at shorter distances. One of many forces acting upon a projectile at specific distances and velocities.
Do I believe this? Sure. Do I also believe the other six million other complex ‘noise generators’ are working on the bullet at the same time? Sure.
There are many valuable contributions above, I appreciate all whom share their wealth of knowledge and first hand experience. Thank you.
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