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  #1  
Old 10-30-2011, 04:51 AM
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Twist rate and bullet weight


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What is the correlation between twist rate and bullet weight? Is it the faster the twist rate the heavier the bullet or is it the faster the twist rate the lighter the bullet?
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Old 10-30-2011, 05:43 AM
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The longer the bullet the more twist required to stabilize the bullet. Weight does not affect stbility, but length does.
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Old 10-30-2011, 06:05 AM
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Quote:
Originally Posted by jwp475 View Post
The longer the bullet the more twist required to stabilize the bullet. Weight does not affect stbility, but length does.

Yep.

Think baseballs and footballs.

A knuckleball baseball (round with zero spin) will fly nearly perfectly straight. The longer, oblong football has to be thrown in a spiral spin to hit its target.
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Old 10-30-2011, 06:32 AM
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A knuckle ball will not fly straight and it's unpreditable andl radical movement is what makes them so difficult to hit
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Old 10-30-2011, 11:35 AM
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Quote:
Originally Posted by jwp475 View Post
A knuckle ball will not fly straight and it's unpreditable andl radical movement is what makes them so difficult to hit
Point is a proper spin is determined by both weight and length but of the two the length is the more critical dimension. We have threads in our archives with links to articles that explain it well depending on how technical you want to get. We use bullet weight in our discussions because a heavier bullet is usually a longer bullet but there are notable exceptions, a long spitzer boat tail bullet made out of copper can weigh the same or less as a considerably shorter bullet in the same caliber if it is lead, flat based and round nosed. Now you know why we say try a mix of bullet weights and styles to determine which one your gun likes. The science will get you close but trigger time and paper groups will tell you the truth.
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  #6  
Old 10-30-2011, 12:02 PM
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so, how does this apply to roundballs, you know, if length is the only variable ??

eh? Grizz
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  #7  
Old 10-30-2011, 12:07 PM
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This should be of interest for this subject:

http://www.shilen.com/calibersAndTwists.html

Last edited by EDip; 10-30-2011 at 12:10 PM. Reason: incomplete
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Old 10-30-2011, 01:59 PM
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Originally Posted by Griz View Post
so, how does this apply to roundballs, you know, if length is the only variable ??

eh? Grizz

Length determines tha amount of spin needed. Case in point the 244 Remington would only stabilize a 100 Rn bullet the spitezer in the same weight was too long to stabilize. The 243 would stabilize the 100 grain spitzer and out sold the 244. Remiington re-introduced the 244 as the 6mm with a quicker twist in-order to stabilize the longer bullets.

The same with the all copper bullets they can be made longer for the same weight and require as faster twist to stabilize
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Old 10-30-2011, 02:36 PM
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Originally Posted by jwp475 View Post
A knuckle ball will not fly straight and it's unpreditable andl radical movement is what makes them so difficult to hit
It was a layman's analogy.

I would, however, invite you to watch my dad (72 years old) crack the mitt with his knucklers. They dance a little. But they still fly straight......hence he can throw strike after strike. The biggest difficulty of hitting a KB is the illusion that it's going way slower (you can read the "Rawlings" on the darned thing) than it really is. Kinda like a changeup on crack.

But I digress into a near hi-jack. Apologies to the OP.
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Old 10-30-2011, 06:04 PM
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It's length, velocity, and weight (well, mass, really). In that order, I believe. Not sure that the weight counts for much but I believe it counts for a little.
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  #11  
Old 10-31-2011, 07:37 AM
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Of course weight matters . You can't make a bullet longer without making it heavier too .( like materials used of course ) You can't compare a monolithic bullet to a cup+core bullet . I have not heard that Barnes bullets require more twist rate than others . Now you would think that a longer bullet would stabilize easier as it engages more rifling than a shorter one . Isn't that why we don't shoot big round balls anymore ?
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Old 10-31-2011, 09:02 AM
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Quote:
Originally Posted by Griz View Post
so, how does this apply to roundballs, you know, if length is the only variable ??

eh? Grizz
Basically, If I understand it correctly, to stabilize bullet you basically have to spin it fast enough so that it will stand on its nose on a table.

A round ball doesn't need much.

Basically all you are doing with a round ball is preventing a spin axis that is anything other than one parallel to the bore. If the round ball comes out of the barrel with a spin axis other than parallel to the bore the magnus effect will make it curve like a curve ball.
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Old 10-31-2011, 03:46 PM
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Originally Posted by oneoldsap View Post
Of course weight matters . You can't make a bullet longer without making it heavier too .( like materials used of course ) You can't compare a monolithic bullet to a cup+core bullet . I have not heard that Barnes bullets require more twist rate than others . Now you would think that a longer bullet would stabilize easier as it engages more rifling than a shorter one . Isn't that why we don't shoot big round balls anymore ?

Barnes makes their bullets to be shot from factory twist rifles and thus they are no longer than many lead core bullets. Mono metal bullets can be made longer and the same weight lead core by streamlining the ogival and thus need a higher spin rate to stabilize

A bullet could be made of all aluminum to be much longer than a lead core bullet and would therefore need a higher twist rate to stabilize
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Old 10-31-2011, 04:33 PM
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well, it's something new learned for me. I knew that our Marlin 1894 couldn't stabilize the 325s I shot for years in my handguns, but I related it to weight rather than length, which are somewhat proportional for the same substance, but different when you change bullet composition.

I don't think of that one as being a very long bullet. But the Marlin chucked them out sideways and they whizzed off at various angles even though they were 100% deadly from the revolver.

What about designs that have less rifling contact. Ranch Dog's bullets and some other cast bullets have large lube grooves that reduce the amount of bullet contact with the rifling. Does this in turn "shorten" the bullet for the purposes of spin stability?

very interesting.
thanks for the info

Grizz
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  #15  
Old 10-31-2011, 04:49 PM
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It's the total length of the bullet, end to end, not how much of it touches the riflings.
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  #16  
Old 10-31-2011, 08:51 PM
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Quote:
Originally Posted by MikeG View Post
It's length, velocity, and weight (well, mass, really). In that order, I believe. Not sure that the weight counts for much but I believe it counts for a little.
I believe that to be correct, sir. I haven't looked at the formulae in ages, but I can tell you this without question:

1. Higher twist rate causes a loss of velocity because more of the forward energy is converted into rotational energy;
2. A longer or heavier bullet requires higher twist to stabilize;
3. Heavier bullets are usually longer, but the added weight adds to the need for additional stabilization (higher twist rate);
4. The higher the velocity, the more spin you impart to the bullet, and the more stabilization is realized, because the lans and grooves grip the faster bullet and spin it faster until it leaves the muzzle;
5. Too tight a twist with a light bullet will cause a point of impact issue (think a slider as a baseball pitch and how the scored bullet interacts with the air). The further the bullet goes, the more the spin changes the trajectory and thus the POI;
6. See point #1, the tighter twist, though causing a loss of velocity, helps to stabilize the bullet downrange.

Now for personal experience. I once had a T/C Encore Pro Hunter 15" in .243 Win. The barrel had a 1:11 twist rate. The ONLY "big game" bullet I could get to stabilize was a Sierra BTHP in 85gr. I had 100gr partitions loaded, but they would keyhole at 50 yds. Not good. I ended up loading them for my dad's Model 88, and he loves them, but the 1:11 of the .243 was not good for deer and hogs in the 15" barrel, which is what I wanted the pistol for. I ended up going with a 7mm-08 barrel for my setup. It has a 1:9 twist. It will stabilize up to (at least) a 150gr partition AND a 139gr SST (super long) because of the higher twist. I load close to max pressure on all of my loads for the T/C to ensure I can push them fast enough to stabilize. In fact, I am right now pushing a 120gr .284 TTSX out of it at over 2900 fps, and I'm getting 1.5" groups from a 140gr Partition at right around 2800 fps. A little flattening of primers, but no pressure signs in the cases. Hope this helps.

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Last edited by DCAMM94; 11-01-2011 at 05:45 AM.
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  #17  
Old 11-01-2011, 03:41 AM
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I have noticed two distinct situations where the length, velocity and weight of a projectile had a large influence on accuracy. They both taught me something about how important rate of twist is when it comes to stable bullet flight.

The first one was my dad's Winchester 870 slug barrel. I forget the exact rate of twist his barrel has, but we discovered that it would not shoot the heavier slugs very well. After figuring out the exact ROT in his barrel and what weight projectile it was meant to stabilize, he purchased some sabot rounds and they shot much better. I "think" the barrel had a relatively slow ROT, which is why the much heavier true slugs would not fly well from it. (I also learned that there is some kind of sliding scale where the diameter of the projectile changes optimum ROT?)

The second lesson learned was with a wildcat 35 caliber I've played with for the last couple of years. It is a rechambered H&R 357 Magnum barrel that has a 1:16 twist. I loaded up some 200gr FTX bullets; the ones used in the 35 Remington, which is a very close approximation of the performance I get from my gun. I discovered that the FTX bullet was key-holing at 100 yards, with the groups really opening up from the tight groups I saw at 50 yards. Only by driving the 200gr FTX at 2200fps (max for my cartridge) was I able to maintain good groups at 100 yards. I found that the 180gr Hornady SSP bullet grouped much better, even if the velocity was only 2250fps, which was a middle charge of powder.

Then came the revelation: I loaded up some 200gr Remington Core-lokt RN bullets. These were the same weight as the FTX bullets, but the same LENGTH as the 180gr SSP bullets. The Remington bullet is considerably shorter than the FTX bullet from Hornady. The 200gr Remington CL RN bullets shoot very accurately from my barrel, at 100 yards, even when only running 2150fps. Length of projectile really DOES matter!

I have stuck with the 180gr bullets as my hunting load because they continue to be stable at 200 yards, whereas the heavier (slower) bullets begin to wobble and lose accuracy at longer range. This experience has made me wonder about the old 35 Remington because it also has a 1:16 barrel. Maybe the reason it has always been great for woods ranges is because it just doesn't drive the normal 200 grain bullet loaded for it fast enough to remain stable past 150 yards? My experience seems to suggest this, anyway.

MikeG hit it right on the head with his post. Length, then velocity, then weight...in that order.
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Last edited by broom_jm; 11-01-2011 at 03:45 AM.
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