Effect of Defects in the Bullet

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  • Last Post 17 October 2011
codarnall posted this 11 September 2010

A very interesting experimental result was emailed to me. It was a classic demonstration of defects of the bullet on accuracy. The gent didn't care to publish the results for various reasons. I learned the hard way teaching physics lab to freshman and sophomore university students. It was the first year to let the student solve problems experimentally of their own design. What a waste of time. Torque to open a door knob, how do you do that?

The sum total of the tiny masses vectorially of the bullet starting from any reference point it will always get to the same point in the object, the cg, here a bullet from your pick of a reference point. Mine may be a different point say the nose. One could simply hang one like a plumb, but that's only one axis. Hang from a different hook or point the intersection of the string line will go the the CG of the object. The more points you hang the object the better the accuracy in finding the CG experimentally.

Real marksman, reloaders, gun nuts, have figured this out a long time ago, that is you shoot uniform bullets, weight dimension etc. Bullets have what are know as static and dynamic stability. A bullet with a CG off it axis is out of balance statically, some revelation right! Missiles and rockets are drag stabilized like the arrow with feathers at the rear. Bullets we shoot are spin stabilized.

So now with defects we have a little off center line problem. The amount off center will determine how far the bullet will be thrown from the bore's axis. Bullets can be made to have small groups around the points of the clock if the defect is made to leave the barrel at the same orientation each time. This has been demonstrated in laboratory settings and the agreement with the digital models is excellent.

It's like a little man inside the gun with a sling shot releasing the CG in the direct of rotation at the muzzle. The length of the string is how far off axis the CG is. I just had to put my hat on from a different life.

Charlie

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tturner53 posted this 12 September 2010

I knew it! I'm so glad I'm not the only one who has a little man inside his gun!   EDIT: I deleted my smardass remarks. My sick sense of humor is often not that funny to anyone but me.

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JetMech posted this 12 September 2010

codarnall wrote: So now with defects we have a little off center line problem. The amount off center will determine how far the bullet will be thrown from the bore's axis. That reminds me of a post by Ed. He took some bullets cast by a top CBA competitor and sectioned them. It was found that they had tiny air pockets in them, but they were evenly distributed close to the longitudinal axis of the bullet, thus minimizing their affect on accuracy.

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hunterspistol posted this 12 September 2010

:coffee     All that is true, given the bullet is up to speed on a rotational axis.  In an experiment published in The Art of Bullet Casting, defects were intentionally induced at different points on the bullet.  Apparently, the base of the bullet made the most difference because, this is where thrust is first applied.  So, the back wall the little man is pushing on, needs to be almost perfect to bring the bullet up to speed, the point at which all these variables apply.  If the base isn't near perfect, then the little man with the slingshot is throwing your CG against the walls of the barrel before it even exits the muzzle. And then things get wild!

     Tim, yeah, digital models, wow.

 Ron 

   PS: Don't get me wrong, I find physics vastly interesting. We should have coffee.

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codarnall posted this 14 September 2010

That's in essence was why I posted the note. It stemmed from seemed to be uniformly made and seated gc's oppose to those likely not. The groups were significantly different. Charlie

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Mnshooter posted this 14 September 2010

Interesting observations except for one point that I have had another discussion about. You stated that arrows are drag stabilized like a rocket. As a 3d competitor with some success and an old time arrow maker I can tell you that some do so but it is easier to get an arrow to shoot, especially with broad heads, if you spiral fletch it to get it to spin. Some claim the concept of rifling was due to that small fact in that they were making arrows spin for a couple of thousand years or so before gunpowder was heard of. A staight fletched “drag stabilized” arrow does not stabilize large two blade broadheads. the modern high speed 3D shooters use the principle because straight fletched arrows with small feathers do not lose velocity as quickly. That is with expensive almost perfect arrows, which illustrates your point about bullet perfection. Those of us that shoot more imperfect wooden arrows have found that we can make more usable arrrows by fletching them so that they spin like a bullet. Your center of gravity point is also demonstrated in that some pretty crooked arrows will look like they are doing a gymnastic routine on the way to the target, but seem to hit where you point them. I have seen pictures of old timers that made some pretty crooked arrows to shoot for fun that would shoot dead on. The point and the nock have to align and again denter of gravity. But we do spin stabilize arrows for accuracy. As a rule if a broadhead does not shoot well you put more spiral on the fletching, if that does not work you use bigger feathers with more spiral, or else go to more feathers to get it spinning. It still kind of illustrates your point. I remember reading in a book where arrows were made broadhead up for game with their rib cage position and broadhead flat for humans for that putpose. My thought was what BS as we get them spinning and they can hit at any position.

Mnshooter

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codarnall posted this 14 September 2010

I am guessing now. An excellent point. Pitch and yaw now play a part in the cp cg delta forces in the arrow. The spin of the arrow I am guessing will now tend to average these forces out. I have no doubt what would happen if the fletching came off. Too, it's seems obvious why trick shooters don't use broad heads. Charlie

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billwnr posted this 14 September 2010

So...we accept all cast bullets have defects. I thought the main thing was to have the cast bullets hit what you aim at.

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JetMech posted this 14 September 2010

It's been a few years since I have read it, but Mann's Flight of a Bullet From Powder To Target had some very interesting experiments along this line. If I remember correctly, many bullets fly a corkscrew trajectory due to imperfections. My take on it was that I couldn't cast perfect bullets, so component orientation would prove beneficial, as they would all start their corkscrewing at the same point and, therefor, minimize the dispersion on target. It seems to have less effect as my casting skills improve.

Another thing to consider is the effects of the wind on the bullet during it's imperfect flight. The wind causes the bullet to drift but also to deflect, the drift being a direct result of the wind, but the deflection being a result of both the wind and the bullets angle to the relative wind, depending on those defect's effect on the bullets flight. Just my ideas on a complicated subject I have no particular expertize in, just opinion and observation.

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codarnall posted this 14 September 2010

Sorry but I've seen some ridiculously small groups, not my me, but some have figured out these black art to perfection. My hat is off to them. Half MOA is not an accident! Charlie

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Mnshooter posted this 15 September 2010

codarnall wrote: I am guessing now. An excellent point. Pitch and yaw now play a part in the cp cg delta forces in the arrow. The spin of the arrow I am guessing will now tend to average these forces out. I have no doubt what would happen if the fletching came off. Too, it's seems obvious why trick shooters don't use broad heads. Charlie Surprisingly, with field points, loss of a fether often does not efftect them much as long as they spin.  Things today are changing and they are using very slim arrows with straingt fletching, but it requires the use of trigger releases and expensive arrow rests.  The broadheads are also vented. The effects caused by finger release and other little nuances in the long bows and recurves I shoot seem to be more negated by the spinning arrow.  Especially in what is termed “recovery".  We had a discussion on another forum about smoothbores, as flintlock fowlers are getting fairly popular.  I have one and found that by really paying attention to detail, such as loading the ball parting line up every time, I could get some pretty good results at 75 yards.  Easily minute of deer.  Discussions got started because some think that since golf balls can be driven farther because they have dimples, they should dimple the smoothbore round ball. Golf balls spin when hit and get an aerodymanic lift similar to an airplane wing due to the dimples.  Smoothbores do not spin, but some seem to think you can still apply the principles. To me if could get the “perfect” roundball out of the perfect barrel shot at the same speed, you should have a very accurate combination.  Its the imperfections that cause the problems which get amplified in a smoothbore.

Mnshooter.

 

 

 

 

 

 

 

 

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codarnall posted this 15 September 2010

No mangus effect (sp), bernoulli etc. The dimples on the golf ball were a wind tunnel result that indicated much less drag. Stability is defined as returning to the previous existing state before being perturbed. Nothing is perfect. The perfect ball will spin somehow some way unless it's in a vacuum. Praise riflings.

Your now straining my brain. Charlie

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Mnshooter posted this 15 September 2010

I kind of thought that sooner or later a smooth bore will go south due to picking up a spin. what I will do is to try to stay more on track now and talk about how some try to minimize imperfections. A 1000 yard BPC competitor mentioned that he dimples his mold so that he has a small point of reference so that he can load his bullet into the case the same way (the dimple is very small and is easily removed). He also notches the back of the case so that he loads it the same way into the breech and as an index for the bullet and uses fire formed cases. Molds do not throw perfectly round. For instance my roundball mold for my smoothbore is .002-.003 out of round. Some bullet molds the same. Yet surprisingly some claim fair extreme range accuracy with these bullets. Dollar Bill touched on this. Rifling rate also enters in as some of the heavier bullets over longer distance start to de-stabilize, although up close you would not notice. Some have to correct through muzzle velocity others have used faster twists. One point on a discussion of accuracy is that accuracy has to be defined in distance as well as group and purpose. I believe it was Frank Marshall that talked about hunting loads and stated that one does not need “to be able to shoot cockroaches at 100 yards to get a deer". Benchrest shooters are never satisfied but it is great to ahve them because their research often gives some of us great pointers for attaining “practical accuracy".

Mnshooter.

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hunterspistol posted this 16 September 2010

Benchrest shooters are never satisfied but it is great to ahve them because their research often gives some of us great pointers for attaining “practical accuracy".      No truer words have ever been spoken, amen!

    Ron

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billglaze posted this 28 March 2011

Just as a matter of information, the dimples on a golf ball tend to break up a “boundary layer” that promotes drag. On some jets, we have what are called “vortex generators” on the upper surface of the wings. These are small wings, about the size of a postage stamp or a little larger, mounted at 90 degrees to the top surface of the wing, with their chord line parallel to the line of flight. They create a vortex that trails out behind them along the top surface of the wing that diffuses the boundary layer of the wing, reducing drag and permitting a fuel savings at cruise speed.. They work well at a relatively high trans-sonic velocity--above about .8 Mach.

In theory, there's no difference between theory and practice. In practice, there is. My fate is not entirely in Gods hands, if I have a weapon in mine.

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HuskerP7M8 posted this 17 October 2011

I realize this is a very old thread, but I've just seen it and thought some might be interested in a similiar experiment I've done several times over the years.

Link: http://landy-smallboreprecision.blogspot.com/2011/04/cg-offset-effect-on-smallbore-bullets.html

Landy

“In God we trust; all others must bring data.” “Without data, you're just another person with an opinion.” “If you can't describe what you are doing as a process, you don't know what you're doing.” “It is not enough to do your best, you must know what to do, and then do your best.” W. Edwards Deming (October 14, 1900 - December 20, 1993)

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