TWIST, STABILITY, INSTABILITY, TIPPING AND KEYHOLING

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joeb33050 posted this 22 December 2018

 

TWIST, STABILITY, INSTABILITY, TIPPING AND KEYHOLING

 

I was led to understand that barrels were rifled to increase accuracy; and that there was a “twist”, measured as the number of inches for one revolution of the bullet. A 10” twist barrel turns the bullet one revolution in 10 inches of forward travel. And there were two corollaries; at too slow a twist, instability happened, and at too fast a twist, accuracy suffered. Opinions varied, as they will.

 

Some of the twist rate formulas use bullet length and velocity in the calculations. I have 22 caliber barrels in 14”, 12” and 9” twist, in 223 and 22-250.

 

Reducing velocity and increasing bullet length should produce instability; tests are under way.

 

The .22 Nosler Varmageddon 40 grain, .665” long bullet has a Greenhill minimum twist of 11.4”.   The 223 Iffland barrel has a twist of 14”; so we would expect instability.

 

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joeb33050 posted this 22 December 2018

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joeb33050 posted this 22 December 2018

Note that none of the twist formulas work-I keep looking for the mistake, and can't find it. I need some help here!

None of the bullets showed tipping or keyholing; EXCEPT my record notes: TIPPING at 7 and 8.5 gr of Titegroup in November 2017.

Two groups were fired with 3.5 and 3.0 grains of Titegroup on 12/21/18. Both 3.0 groups were fine, the second 3.5 group had 2 bullets miss the paper entirely. No tipping.

 

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joeb33050 posted this 22 December 2018

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joeb33050 posted this 22 December 2018

This is mv vs. group size, with the 3.5 grain/1404 fps avg set at 2.000 inches-arbitrary.

As powder charge is reduced, the risk of a stuck bullet increases, and we run into the speed of sound. The expected tipping/keyholing/wild shots haven't shown up-except for the 2 missing shots.

There seems to be a u-shape to the data, some of the other bullet/barrel combinations  seem to show this.

Unless I've made a/a series of mistakes, nothing horrible happens when twist formulas are not adhered to.

Workbooks for barrels/bullets in 223 and 22-250 are available on request.
 

 

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RicinYakima posted this 22 December 2018

"Unless I've made a/a series of mistakes, nothing horrible happens when twist formulas are not adhered to."

Joe, I hope that this is a "Eureka" moment. I believe your math is perfect. All the formulae are trying to make an engineering answer for observed phenomena. So the five things you are looking at are not all related to the rotational speed of the bullet as the ONLY facture. There is something else we are missing in our considerations. Ric

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Scearcy posted this 22 December 2018

Joe

I am not an engineer and I pretend to have no unique knowledge that others haven't already mentioned in these various twist rate discussions. However let me be a friendly devils advocate.

There is a fair amount of literature that suggests Greenhills formula  should have a constant of 180 to 200 when being applied to bullets of current manufacture. A constant of 200 brings Greenhill in much better alignment with observed behavior.

The velocity adjustment is a cube root and as such is not a robust factor without large changes in velocity.

Forgive me if I should already know this. Are your groups recorded at 100 yards? If so would the data differ significantly at 200 yards and beyond. It seems reasonable to assume that there is a time factor involved in the emergance of indicators of instability.

I would argue that your data is displaying evidence of instability at the lower velocities. Not only are the average group sizes significantly larger but the variance in the group size (standard deviation perhaps) is also much larger below about 2,000 fps.

There is no Eureka moment in my observations but perhaps they are helpful.

Merry Christmas

Jim

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joeb33050 posted this 22 December 2018

Jim;

Yes, all 5-shot 100 yards.

Greenhill with a constant of 200 has a minimum twist of 15.2" with a .665" long bullet; and has a max bullet length of .723" with a 14" twist. So, Greenhill works with a constant of 200, an accepted constant. 

I vary MV for different bullet weights/lengths because it's the only thing I CAN vary. I need an adjustable twist barrel. Ken?

The Iffland 14" twist barrel with 53 gr. bullets starts to be unstable ~7 grains of Titegroup; but at 8.0 and 8.5 grains, primers get uncomfortably flat.Maybe Blue Dot?

The goal is to get instability well > speed of sound, and take apart what happens in quarter grain powder increments.

I can make bullets shoot wildly, off the paper, keyhole and tip, but not the same bullet/barrel. And, it's looking as though my imagined sequence of wild/keyhole/tip/stable may be truly imaginary.

Or, I could stay home and watch daytime TV.

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RicinYakima posted this 22 December 2018

Joe, Don't do that! Then what would I think about while I'm napping with dog in the afternoons? I've had a lot of fun trying to make sense out of all of this. Ric

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Scearcy posted this 23 December 2018

Joe, just because you are not proving what you expected to prove doesn't mean we aren't learning anything.  Jim

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Ken Campbell Iowa posted this 23 December 2018

joeb ; ... adjustable twist ...  heh, fits right in with your stability test results ... which both amuse and scare me at the same time.

but i have found a safe space by clinging to a box of 22 match ammo ...  you know, 40 grains at 1080 fps... and stabile in a 16 twist for as far as the eye can see ...  even in a pencil barrel ruger 10-22 i bet ...

3 inch++ groups from a 40 gr nosler at 2000 fps ? .... "" say it isn't so, joe "" 

... makes a guy want to believe in Scatter Claus ...

ken

 

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joeb33050 posted this 23 December 2018

One of the interesting finds in the Jacketed Bullet Test is that 40 grain Nosler Varmageddon bullets shoot at least as accurately at 100 yards as the best 22 rf cartridges/guns. The Iffland barrel on a Savage M12 action averaged .755" for 52 5-shot 100 yard groups, charges of Titegroup between 6.5 and 8.5 grains, mv between 2273 and 2698 fps. 

My BSA Martini 12/15 and International rifles will infrequently shoot < 1", but average ~1.2" with inexpensive ammunition. Perhaps Eley Tenex, at ~ 30 cents a shot, might consistently shoot <1"; but the Nosler bullets cost ~16 cents.

I think it's safe to say that cf 22 guns/reloaded ammunition can shoot at least as well, at 100 yards, as the better rf 22 guns/ ammunition; and at a much lower cost. 

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Scearcy posted this 23 December 2018

I have mentioned this before in other threads but it may also have value here. 2 summers ago I was shooting a Tikka 243  in the local matches. The rifle has a measured twist rate of about 10.2" (260 MM). My bullet was the NOE 245-105 FN. Greenhill said I needed a 9.4" twist to stabilze the bullet.

I won a few local 100 yard matches with the combination in spite of Greenhill's warning. In late July we shoot a two day match with one full day dedicated to 200 yards. At 200 yards my combo would not keep the hits on a CBA score target. Several bullets that did hit the paper went through sideways.

Last year I used a 243 rifle with a 9 1/4" twist. Using the same load, everything was stabile and accuracy at 200 yards was very good - relative to my ability.

Not all instability is apparent at 100 yards.

Jim

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joeb33050 posted this 23 December 2018

Not all instability is apparent at 100 yards.

Jim

I'm assuming that all instability is apparent at 100 yards AS speed is reduced. Seems to be so; so far.

joe b.

 

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Ken Campbell Iowa posted this 24 December 2018

... is it less velocity at 200 yards ... or just that the marginally stable bullet has been spinning 2.1 times as long and the wobbling is increasing ...  with time ...

ken

 

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Scearcy posted this 24 December 2018

I certainly can't say. The bullet has slowed enough to become trans sonic. Perhaps that is sufficient to completely destabilize the bullet. It is my understanding that the bullets velocity loss would be much greater than its loss of angular velocity. Does this indicate RPMs should still be sufficient - apparently not so. Remember this bullet and load are stable out of a 9.25" twist even when trans sonic. Maybe it simply has more time to wobble.

To further complicate things, the RCBS 245-95-sp remains stable at 200 yards when fired from the same Tikka. Don't be fooled by the names. The NOE bullet and the RCBS bullet are the same weight and exactly the same length. The profile differs of course. My layman's conclusion is that the RCBS design is simply more stabile or easier to stabilize, however you wish to say it.

Jim

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Larry Gibson posted this 24 December 2018

Scearcy

Can you post a side by side picture of both bullet?

If I recall the original RCBS 245-95-FN correctly it has larger lube grooves and a shorter blunter nose, more of a pseudo Loverin design.  That would mean a lower BC than the 243-095-SP.  Thus it lost velocity quicker than the SP cast.  The later re-designed 24-100-FN also has more lube grooves which are shallower and a shorter blunter nose with a lower BC also.  The difference in BCs and different retained velocity can account for the different stability past 100 yards.

LMG

Concealment is not cover.........

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Scearcy posted this 24 December 2018

I'll try to get a picture before people begin to arrive. The RCBS bullet I have is what I would call a scaled down 311299. The NOE has a significant flat nose. I measure one each this morning and the NOE is actually .017" longer than the RCBS.

Jim

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Scearcy posted this 24 December 2018

Here they are. Just for fun I included a sectioned case with the RCBS bullet. The NOE is on the left.

I will be using the RCBS bullet for 3 targets (3 rifles) in the 200 yard coyote hunt. Two of the rifles have 10" twist. We will see if cold weather makes any difference.

Jim

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Redleged posted this 25 December 2018

Jim,

Those are sure pretty! Are you Powder Coating and conventionally lubing? I'll sometimes TL my PC bullets, but hadn't thought to run them through the Lyman 450.

Ed

Growing old is mandatory, growing up, however, is totally optional!

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Scearcy posted this 25 December 2018

Ed

The bullets went through a standard lube sizer. The blue/purple hue is caused by the camera settings. My workshop has bright florescent lights. In order to get true colors in that kind of light, one needs to make a manual adjustment to the white balance setting in the camera. I didn't do that so the colors came out cool (blue). It is a nice color though.

Jim

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