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#6328670 - 06/09/16 05:43 AM Twist Rate and Bullet Stabilization
charlesb Offline
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Registered: 04/07/10
Posts: 1408
Loc: West Texas mountains
I have discovered that though barrel twist is the primary factor in determining how a bullet of a given weight will perform from a rifle barrel, that at least two other factors influence whether the bullet will stabilize well or not.

Maybe three or more.

* Velocity for example. This makes sense when you consider the fact that with a given twist, the faster the bullet is going when it exits the barrel, the higher the RPM it will spin at, in flight. On my 1:12" twist Howa, 55 grain boat-tail bullets stabilize just fine at 3300 fps, but at 2000 fps they drift out and produce much larger groups. In these two cases, only the velocity was different. All other factors were identical. (Same bullets, same gun, same bullet seating depth, etc.)

* Another thing that affects the bullet's flight is the imprint upon the rifling upon the bullets skin, which will directly affect its aerodynamics in flight. I believe that this is often the culprit when one rifle will successfully stabilize a bullet of a given weight, when most other guns of the same twist typically do not.

Here I think about the golf ball... Many times over the years, golf enthusiasts have had the idea that the ball would fly better if those dimples were removed. Slicking up the ball should make it fly more straight and true - but it doesn't, not at all... Smooth golf balls are erratic in flight, and do not travel as far as the standard design with it's dimples all over the surface. At the speeds that bullets fly at, this affect is magnified.

Rifling varies quite a bit between guns, even guns from the same maker. The number of lands, the shape of the lands, the height of them, and smoothness of the rifling all have some effect. Then there is the difference between a clean or dirty barrel, and the difference between a newly sharpened rifling cutter and one that has seen some use. How far the bullet jumps before engaging the rifling, and the amount of throat erosion also affect the outcome.

* The length of the bullets bearing surface, I have been told, also makes a difference. Experienced varminters have suggested that flat-based bullets will often stabilize and fly more true than boat-tails of the same weight. If other factors are putting the bullet close to the dividing line between stabilizing or not, then the amount of bearing surface for a given weight bullet can push the bullet over the line to becoming more (or less) stable in flight.

There are other factors I am certain, such as shape the bullet's ogive, and the length to diameter ratio, but I believe that these I have outlined are the things that most strongly affect bullet stability, and explain why one rifle will shoot a given bullet just fine, while another will not.

The effects are cumulative. If some are there but others are not, then the effect is not going to be so profound.

That's the theory, anyway - and we have no shortage of those! No telling how many other things might have an influence upon the bullets stability in flight.

My old Savage model 25 in .223 with a 1:9" twist would stabilize 72 grain bullets just fine. It wasn't supposed to, but it did anyway. I would hate to have to try to figure out exactly why this was so.
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#6328775 - 06/09/16 07:51 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
ChadTRG42 Online   happy
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Registered: 09/16/09
Posts: 9420
Loc: Lewisville, TX
There's about 100 things that influence how accurate a round is. If a bullet is stable, even minimally, it can still shoot very accurately. For example, a lot of your bench rest shooters will run a very slow twist and their bullet will have a 1.0 to 1.1 stability factor. A 1.4 to 1.5 or greater is what is generally needed for full stability with no reduction in the BC of the bullet. For the 100 and 200 yard shooting that these bench rest guys shoot, BC isn't a huge factor in their tiny groups.

Also, take the 270 Win shooting the new Nosler 150 grain Long Range Accubond (which is what I have been shooting some for my personal rifle). Out of the standard 1:10" twist at ~2900 fps, the stability factor is a 1.19, which is marginally stable. This bullet can be made to shoot very accurately out of the 270 Win. But the cost is a reduced BC because of lack of full bullet stability (about a 4.5% reduction in BC.)

On your 22 caliber shooting the 55 grain BT, (I'm assuming 22-250, maybe a 223 if pushed hard, but either one will work for stability factor), I couldn't find a 55 grain BT, so I'm running the numbers with a 52 grain Nosler BTHP. At 3300 fps, I'm showing a 1.42 stability factor. When you drop it down to 2000 fps, the stability reduces to 1.2 stability factor. The bullet is still stable at 1.2, but is below the desired 1.4 or 1.5 and will not have 100% of the BC of the bullet, and will have a slight reduction in BC (9% less to be exact). Even at 1.2, the bullet can still be shot very accurately with the right load. What I think is the main issue in lack of accuracy, is not pushing this reduced set up with the most optimum powder selection for a reduced load. Most rounds shoot best at or near top pressure. The right powder used at this max area should work well for accuracy. (If it's a 22-250, something in the Varget burn speed should do well). Now if you reduce the load to get 2000 fps and keep using Varget (or similar slow burning powder), the load becomes very inefficient and Varget is a much too slow burning powder at this set up. To help gain back this efficiency, you will need a much faster burning powder. I would estimate something in the H4198 or a touch faster as the burn rate, and this will help with getting the round back to a more efficient round.

The same thing happens with a subsonic round and selecting the right powder. You can get most any powder to shoot at a slower speed, below the 1100 fps velocity. But the issue becomes how efficient is it, to keep your pressure and velocity consistency. The slower powders as a subsonic will yield velocity swings as high as 200 fps variation, which I have seen when testing subs with a slow burning powder. As most reloaders know, Trail Boss works as a good subsonic powder (for most applications) because it is a fast burning pistol powder, and it is a fluffy powder that fills the case up more. Both these 2 factors help make the subsonic round more efficient, getting an even pressure and keeping your velocity consistent.

In my opinion, the biggest factor (major factor) to getting an accurate round is selecting the right powder burn rate for the cartridge and bullet combo used. For example, the powder used in a 300 Win Mag when shooting a 125 grain bullet would be totally different than the powder used when shooting a 220 grain bullet. After that, the seating depth, case prep, powder charge consistency, and all the other detailed items are a minor factor, but still important.
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#6328801 - 06/09/16 08:07 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
ChadTRG42 Online   happy
THF Trophy Hunter

Registered: 09/16/09
Posts: 9420
Loc: Lewisville, TX
Originally Posted By: charlesb
My old Savage model 25 in .223 with a 1:9" twist would stabilize 72 grain bullets just fine. It wasn't supposed to, but it did anyway. I would hate to have to try to figure out exactly why this was so.


Point in case. I don't know of a 72 grain bullet, but I'll calculate using a 75 grain VLD in this case. With a 1:9" barrel twist, a 75 grain VLD at 223 Rem speeds of 2700 fps, shows a 1.11 stability factor. The bullet is stable, but marginally. The bullet can still be loaded to shoot well and be accurate. Since the bullet is not adequately stabilized above the 1.4-1.5 stability factor, the BC will be reduced by 12%. The reason the BC is reduced is because the bullet will have some yaw and slight movement in flight. Think of a spinning top you played with as a kid. The faster you spin it, the more tight the top spins with no wobble. Think of this tight spin with no wobble as a stability factor of over a 1.5. As the top slows down, you will see some wobble begin to happen. But during this wobble, the top is still spinning and is stable, but marginally stable (with a stability factor between 1.0 to 1.4). This is similar to a bullet as it is flying through the air. Between the 1.0 and 1.4 (or 1.5) stability factor, there is some "wobble", which reduces the BC some, but the bullet is stable. That's the best way I can explain it.

I am referring the stability numbers using the Berger calculator (below). I use it often, and I think it is a valuable resource. Bryan Litz with Berger Bullets, who has a degree in Aerospace engineering, has done a lot for our sport to help understand the science of bullet flight. I read all of his books, and have spoken to him many times about topics similar to this.

http://www.bergerbullets.com/twist-rate-calculator/
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#6328815 - 06/09/16 08:28 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
ChadTRG42 Online   happy
THF Trophy Hunter

Registered: 09/16/09
Posts: 9420
Loc: Lewisville, TX
Another thing to remember, bullet stability is mainly based on the bullet length, not the weight. Yes, heavier bullets are normally longer. But a .308" bullet that is 200 grains, and is a flat base design with a blunt SP will be much more stable in the same twist than a 190 grain bullet that is a boat tail hollow point design that is a much longer bullet (compared to the 200 grain).
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#6328856 - 06/09/16 08:57 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
Judd Offline


Registered: 01/22/09
Posts: 10757
Loc: Sachse, TX
An early morning lesson in bullet stability, nicely done Chad.
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#6328875 - 06/09/16 09:07 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
charlesb Offline
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Registered: 04/07/10
Posts: 1408
Loc: West Texas mountains
Really excellent information.

I have bought the .223 Hornady 55grn FMJ BT in bulk. Because I have so many of them, I try to use them whenever I can.

One thing about the new all-copper bullets that interests me is that they will tend to be longer than lead bullets of the same weight. I suppose there is also more drag, but it surprises me that I haven't heard more about them becoming popular with long-range target shooters. I suppose the extra length would make them more sensitive to cross-winds as well. More sail area for the wind to catch.

I've decided to go from a 1:12" twist to 1:9" on my .223 target/varmint gun. Mainly because the reduced 55grn loads I prefer will be more stable at 2k fps.

I tried H4895 for my light loads and got vertical stringing, probably from inconsistent ignition. I never get this with AA5744 though, so I have stocked up on this powder for my reduced loads for the .223.

In a rifle chambered for 270WSM, reduced loads at 2100fps with 130 grn bullets using AA5744 gave me the best accuracy that I have obtained with the light loads so far, the best five-shot group at 100 yards being .317" as far as I could determine with a dial caliper. There was nothing special about the bullets, they were Hornady 130grn spitzers.

Maybe I"m wrong, but after my experience with the 270WSM, I'm wondering if I might be barking up the wrong tree with the .223 case, and might actually do better with reduced loads for a 22-250, very similar in shape to the 270WSM.


Edited by charlesb (06/09/16 09:12 AM)
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#6328897 - 06/09/16 09:27 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
ChadTRG42 Online   happy
THF Trophy Hunter

Registered: 09/16/09
Posts: 9420
Loc: Lewisville, TX
Originally Posted By: charlesb
I have bought the .223 Hornady 55grn FMJ BT in bulk. Because I have so many of them, I try to use them whenever I can.

I've decided to go from a 1:12" twist to 1:9" on my .223 target/varmint gun. Mainly because the reduced 55grn loads I prefer will be more stable at 2k fps.


Yes, the 9" twist will help increase the bullet stability. At 12" twist, the stability is 1.16, which is marginally stable. At 9" twist, the bullet is 2.06 stability factor, which is more than enough stability at 2000 fps. The only issue with this bullet at 2000 fps, is at 240 yards, you fall below the 1340 fps, which is now beginning the transonic region of flight. This is where bullets begin to get squirrelly (more unstability and possible begin to yaw more and tumble). There is not a 100% method of calculation to make sure a bullet will transition well in this region of transonic flight, so it's a shoot it and see how it does. But 240 yards and in, you should be fine with the reduced load.

I've always thought that the ~1100 fps area (above the speed of sound) was the important number to pay attention to when a bullet gets close to becoming unstable and subsonic. But talking to Litz, that number is actually right at 1340 fps, which is the beginning of transonic flight, where the bullet begins to become unstable and cause turbulence behind the bullet.


And you are right-on with the faster burning powder with the reduced load.
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#6328912 - 06/09/16 09:37 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
bside Offline
Bird Dog

Registered: 06/10/14
Posts: 314
Loc: DFW
If anyone is interested in playing around with the factors: http://www.bergerbullets.com/twist-rate-calculator/

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#6328913 - 06/09/16 09:38 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
FiremanJG Offline
THF Celebrity

Registered: 12/16/08
Posts: 17893
Loc: Wolfe City, TX
Originally Posted By: charlesb


One thing about the new all-copper bullets that interests me is that they will tend to be longer than lead bullets of the same weight. I suppose there is also more drag, but it surprises me that I haven't heard more about them becoming popular with long-range target shooters. I suppose the extra length would make them more sensitive to cross-winds as well. More sail area for the wind to catch.


Depends on where that length is. If it is on the bearing surface, yes, worse in the wind. If it is on the ogive forward, BC was increased, so better in the wind.

One thing an extreme range shooter and I visited about was monolithic bullets tend to do worse when they're approaching trans-sonic. He did better with a SMK than a lathe turned solid.

Lead is cheaper than copper, as well.
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#6329005 - 06/09/16 10:57 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
JTPinTX Offline
Woodsman

Registered: 04/01/15
Posts: 182
Loc: TX Panhandle
As far as monometals go, if you read Litz book Applied Ballistics, he states that most of the BC's for monometals are significantly over-stated. He believes that this is because the G1 and G7 models do not fit the monometals correctly. It has to do with increased wetted area (surface drag), and also that the driving bands created additional friction that is not accounted for in the drag model. The ones he tested had to have the drag model modified to give accurate drop values. Also, I do believe I remember the book stating there were some differences around the trans-sonic zone as well.

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#6329029 - 06/09/16 11:16 AM Re: Twist Rate and Bullet Stabilization [Re: JTPinTX]
FiremanJG Offline
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Registered: 12/16/08
Posts: 17893
Loc: Wolfe City, TX
I thought I wanted to run solids out of a .375 Chey Tax based on G-1 BC reported in the .9 plus area. I've learned what you stated to be true, and those high end bullets are not all that, as of right now.
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#6329058 - 06/09/16 11:47 AM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
JTPinTX Offline
Woodsman

Registered: 04/01/15
Posts: 182
Loc: TX Panhandle
That was the takeaway I got JG. Those BC's are calculated off the formula, but don't hold up when shot through doppler radar. I personally don't know it to be true, but trust Litz and what he says in his book. IIRC, many of the BC's for monometals were off 10%-20%. And to be accurate, I do believe he stated that he "suspected" it was due to increased wetted area and driving bands, but he wasn't sure. I don't want to be putting out false info there.

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#6329117 - 06/09/16 12:43 PM Re: Twist Rate and Bullet Stabilization [Re: JTPinTX]
FiremanJG Offline
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Registered: 12/16/08
Posts: 17893
Loc: Wolfe City, TX
I don't think that's false. When Litz assigns a BC, I think most of us, by a wide margin, plug in his BC and believe it to be 100% correct.
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#6329256 - 06/09/16 02:20 PM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
JTPinTX Offline
Woodsman

Registered: 04/01/15
Posts: 182
Loc: TX Panhandle
Sorry, what I meant by false information was, that in my first post I implied he said those factors WERE the cause, when in actuality he said he SUSPECTED they were the cause. I believe the information is correct, I just mis-quoted him a bit as to why.

And I am with you, anytime I have a choice of his BC data or data from somewhere else, I plug in his.

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#6329681 - 06/09/16 07:07 PM Re: Twist Rate and Bullet Stabilization [Re: charlesb]
charlesb Offline
Pro Tracker

Registered: 04/07/10
Posts: 1408
Loc: West Texas mountains
I doubt if I would ever try shooting over 150 yards or so with the 2k fps .223 loads. They are intended for cheap-o target shooting at 100 yards, and for situations where a low report and reduced pelt damage would be advantageous for short-range varmint hunting.

So far, I have not tried any of the copper bullets for big game hunting. A lot of hunters swear by them, and show impressive-looking recovered slugs.

I probably will not buy next year's big game rifle for a couple of months yet. In my case, the cartridge is always chosen first, then I look at what I can get for that cartridge. Right now I an undecided between 300WSM and 325WSM. Both have their weak and strong points.

I intend to try some big game loads with the copper projectiles this year. If they behave well at the bench, they may make it out to the field.

Meanwhile, I'll have fun playing with my .223 when the new 1:9" twist bull barreled action comes in. Howa gives a choice between barreled actions with 20" and 24" bull barrels, at the same price. Decisions, decisions...
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