BULLET INSTABILITY

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joeb33050 posted this 2 weeks ago

I assumed that as velocity and/or twist rate reduced, that bullets were stable, tipped, tipped more, then got sideways. During the Jacketed Bullet Test, there were examples where this was not true. Some small groups included tipping and even sideways bullets. 

Maybe we don't know what "unstable" means.

The 68 gr Hornady hpbtm in 22-250 becomes unstable at 5.0 gr Titegroup, sort of. At 1248 fps/5.5 Titegroup, there are no tipping bullets. At 1238 fps/5.0 Titegroup, about half the holes are round, with a group, and half the bullets go through sideways, all over and off the paper.

 

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Ken Campbell Iowa posted this 2 weeks ago

thanks joe ....  sometimes i think we are like the committee measuring different parts of the elephant ...  then sending in mega-reports of what they find ... yet nobody actually knows what an elephant looks like ....

hey:::  what if 2 inch groups IS the elephant ??   

we have gone to heaven and don't know it ??? 

cool

ken

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Larry Gibson posted this 2 weeks ago

Joe is certainly posting some good informational parts of the accuracy "elephant" but we should not forget stability is only one part of that accuracy "elephant".

LMG

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

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RicinYakima posted this 2 weeks ago

Stability is the center of mass of the bullet following a path prescribed by velocity and gravity. The center of form may rotate around this path making yaw and oblong holes in the target. Instability is both the center of mass and center of form varying from the path dictated by the barrel. When center of mass and center of form trade front and rear places during flight, the bullet "turns over" in the air and will no longer follow any predictable flight path.

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frnkeore posted this 2 weeks ago

What Joe is calling, unstable, MAY be, since he is in the transonic range, tipping do to the buffeting in that range.

Look at the graph that I posted in the other thread, you'll see that in the tanssonic range, it takes a increase in rpm, to keep the bullet stable or at least point on, offsetting the buffeting.

The transonic range, is where you do not want to shoot.

Try lowering your velocity to under 1000 fps <(edited) and see if the tipping goes away.

Frank

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M3 Mitch posted this 2 weeks ago

There is some good research going on here, although, let me just say that while sticking a cast bullet in a good barrel is somewhat distressing, sticking a jacketed bullet in a good barrel can be quite distressing, unless you are already set up and experienced at getting them out.  (I'm not, FWIW.)

 

Just sayin'.

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frnkeore posted this 2 weeks ago

Although it's not very inaccurate, this is what dropping into the transonic range at 200 yards, will do on target.

Frank

 

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M3 Mitch posted this 2 weeks ago

In another thread on 30 Carbine, Ed noted that he had a load that shot like 2" at 100 yards, but the bullets tipped.  A similar load with a slightly shorter bullet had all the bullets hitting the target point-on, but the groups are not any better.  And here you are doing something similar with a 22-250, at low velocity.

I would imagine that a tipping bullet has the point of the bullet whirling around describing a circle as it flies forward, at full rotational velocity - but maybe it's not like that at all?  Maybe the tipping comes and goes?  Maybe it's like precession where the point describes a circle at a much slower rotational speed than the twist rate of the bullet?  Like how the Earth's axis describes a circle over several thousand years, and for that matter what about "nutation" where the rotational axis sort of nods in and out?

I'm not coming up with any good easy tests to determine what the tipping is like.

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frnkeore posted this 2 weeks ago

Note that the tipping on my 200 yard target, is point on. The bullets that went through straight, have the basic mepat print, as the severally tipped bullets.

All impacts are single bullets, there are no "doubles", just 10 shots, with 4 in the upper group.

Frank

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RicinYakima posted this 2 weeks ago

Trans-sonic is a very ugly posture. You have a shock wave in front of the bullet, but air touching the sides and or back of the bullet. Avoid it at all costs. I have found the Lyman No.3 Cast Bullet Handbook very valuable in looking for loads for shooting cast bullets at 300 yards. What shoots great at 200 may be all over the paper at 300.

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joeb33050 posted this 2 weeks ago

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joeb33050 posted this 2 weeks ago

So, here we have a bullet being VERY inaccurate at ~1138 fps, then getting more accurate as mv increases toward 1800 fps. 

None of the 4 twist formulas tell us that the bullet is stable anywhere in the actual velocities..

Maybe instability and accuracy aren't related?

What is instability, as forecast by these models? What do instable bullets do?

(I'm beginning to wonder about you knowing about photosynthesis!)

 

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joeb33050 posted this 2 weeks ago

Although it's not very inaccurate, this is what dropping into the transonic range at 200 yards, will do on target.

Frank

 Frank;

Do you know, KNOW, that the bullets got transonic,(whatever that means), as they went thru the target?

Look at your graphs. There is a sharp transition, over very few fps. Not much of a range. I'd expect wider variation than the pic shows. The normal variation in velocity should put some bullets above and some below the skinny region.

I have notes and targets showing YES, tipping at higher velocities; and NO tipping just before wild inaccuracy. Tipping is not, so far, a reliable indicator of instability/wild shots, coming.

 

 

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RicinYakima posted this 2 weeks ago

"Maybe instability and accuracy aren't related?"

That is my hypothesis.

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Larry Gibson posted this 2 weeks ago

"Maybe instability and accuracy aren't related?"

 

I believe I reiterated that in another thread on this subject and gave an example of such. 

We can have very stable poor quality bullets and poor accuracy. 

We can have poor stability with quality bullets and what appears to be "good" accuracy at a given range generally at a relatively short such as 50 or 100 yards. Frank's target example demonstrates that at 200 yards his bullets were losing stability and wobbling. While the accuracy/precision was also going south the bullets were probably not yet completely tumbling but had severe yaw and/or wobble.

If a bullet is unstable at exit from the muzzle with there will be no accuracy or precision.  Even at a very short range such as 25 yards with such we can completely miss a standard size target or the bullets can go through sideways (keyholing).

If a quality bullet is marginally stable there can be accuracy even through the point (short span of range) where the bullet begins wobbling.  The bullet will still may travel some distance before the nutants overcome the dynamic stability of the bullet and it completely tumbles.  Accuracy/precision will get progressively worse as the range increases. 

A marginally stable quality bullet may exhibit stability by making round holes in a target but give poor accuracy/precision at shorter ranges because the bullets have not yet begun to wobble. 

There are other things than stability or loss of stability that affect the flight of the bullet causing inaccuracy.  Some may not want to believe that but it is fact.

"Quality bullet" refers to bullets having the center of mass and center of form coinciding with the axis of the bullet. 

LMG

 

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

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tlkeizer posted this 1 weeks ago

Greetings,

FWIW, my 2 cents worth (inflation, you know).  Some years ago I was shooting a Sheridan Silver Streak, and as I was by a large body of water I thought I would see how far it shot.  There was no concern for bullet going across the water as the river at that point was a mile wide or so.  When the bullet was shot with three pumps of air, the bullet came out fine, and then started to spiral as it got further over the water, beginning about a hundred yards or so if I remember correctly.  So, I tried max load of 10 pumps of air, and the same thing happened only with the bullet going faster, and the spiral grew larger much quicker after starting.  I don't remember if the bullet stayed on track further with the "heavier" load, but I think it did marginally.  Also, out to 100 yards I did not see any tipping with what little I shot it on paper at that time.  What i learned then was that the pellet rifle was good out to 50 yards, okay with some luck out to 100 yards, and past that I should not shoot.  After 60 years it still works great on rabbits and squirrels, though I think it does not meet the requirements for Ed's "bunny guns".  I have had to go to pellets rather than the 5mm bullets as I cannot find them up here.  The pellets so far are doing fine.

TK

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Larry Gibson posted this 1 weeks ago

"the bullet came out fine, and then started to spiral as it got further over the water, beginning about a hundred yards or so if I remember correctly.  So, I tried max load of 10 pumps of air, and the same thing happened only with the bullet going faster, and the spiral grew larger much quicker after starting."

An observation of the "helical spiral" that really does happen.......

LMG

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

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Hornet posted this 1 weeks ago

I saw that helical spiral at a fun shoot a couple of years ago. It was an NEF in .500 S&W with a light practice load the owner picked off the internet. That big 1/2" Gas Check was fairly easy to pick up in the right light and it followed an increasing diameter helical spiral that was hilarious once the mind un-boggled. Point of impact seemed to vary considerably with velocity variations. He had another (medium velocity) load that actually behaved MUCH better. It wasn't as funny though.

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OU812 posted this 1 weeks ago

There are videos on YouTube showing how some pellets wobble in flight. The pellet that fell asleep or did not wobble was always more accurate. The guy mounted and aimed thru camera to demonstrate. I will try to find video.

Here is one video, but not the exact one I was thinking about. He plays back camera in slow motion to watch pellet. Maybe we could mount camera on our scope to watch bullet.

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OU812 posted this 1 weeks ago

I believe he mounted camera at rear of scope to aim and film pellet. This should also work with smokeless loads.

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joeb33050 posted this 6 days ago

53 GR BULLET IN IFFLAND 14" TWIST BARREL. WHERE IS INSTABILITY?

 

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