Asymmetric and Unbalanced Bullets

[dverna]

I have always assumed that the reason we can not achieve the same accuracy with cast (or at least it takes a lot more effort) is that cast bullets are inherently less consistent. There may be small voids, molds may not always close the same way, pouring consistency can cause differentials in fill, mold temperature changes (caused when the cadence is interrupted) affect fill,, alloy may be slightly different just after fluxing than after 50 pours etc etc.

If I understand the RPM theory, it essentially states, that there is a point where some "normal" variation in the bullet causes the bullet to become more unstable than acceptable. The bullet has always been unstable but stable enough to provide reasonable accuracy when velocity (spin rate) was behold this threshold. One this point is reached, the bullet must be "more perfect" - and/or it needs to be loaded with more consideration to the kinds of things we do for benchrest ammo- and/or the gun itself must be better (chamber consistency?)

We go ga-ga over a 3/4" cast group that would be relatively easy to accomplish with jacketed bullets in a hunting rifle. We know that something like a .35 Remington is more "cast friendly" (= less affected by bullet inconsistency) than a 5.56.

The loading techniques that allow us to achieve good accuracy with the more perfect jacketed bullets are not good enough to address the variations that occur in a cast bullet. So, we need to take more care to things like neck tension, bullet concentricity, bullet distance to the lands, etc etc.

This "threshold" will move upwards as we either load better bullets or improve how we manufacture the ammunition.

What I am not sure of - but suspect - is there could be a "harmonic zone". Much like a tire that is out of balance. It runs fine at less than 40 mph (even though out of balance), but shakes excessively at 50-60 mph, yet seems to run better again at 75 mph. When we develop a load we start from the sub 40 mph area and things start to go to hell as we get to the harmonic zone - so we improve the balance of the bullet/load. But we either do not or can not drive the bullet past this harmonic zone. Or it could be the imbalance is so destructive, the bullet can never fly true at any speed past X.

Cabin fever is a funny thing.

Don Verna

[Digital Dan]

Wish there was some way I could snap my fingers and the rank and file instantly understood the mechanics of spin stability and all the baggage that goes with it. It is quite clear that many don't.

[45 2.1]

45 2.1 continually post untruths about what he thinks I believe and say. So let’s take a look at what his untruths are in this thread as he posted them. I will tell you what I really think and say in answer to each of his "talking points".

1. We reduce tolerance stack.

Unknown what he means by this(?) but assuming he means consistency of loading procedures and equipment so the bullet, cartridge and components along with the rifle are consistent and aligned. Larry Gibson

Gear is correct. Keeping tolerances "stacked" correctly will give give some push to the RPM Threshold. Doing both by stacking and slowing the RPM with a slower twist will achieve accuracy at a much higher level of RPM. Larry Gibson

Wow... talk about using new terms and NOT understanding anything about them. I read these and now I have to clean my monitor and keyboard. Don't drink anything while reading things here, it's a lot safer that way.

If we want accuracy, we need to understand what's going on. One can band-aid the real problem and get mediocre to decent results, or one can ascertain and correct the problem and get excellent results. To do the latter, gaining a deeper understanding of the "tolerance stacking" mentioned by 45 2.1 is a good place to start, but it is only a start. Gear

Ha, you were paying attention...............

I have always assumed that the reason we can not achieve the same accuracy with cast (or at least it takes a lot more effort) is that cast bullets are inherently less consistent. There may be small voids, molds may not always close the same way, pouring consistency can cause differentials in fill, mold temperature changes (caused when the cadence is interrupted) affect fill,, alloy may be slightly different just after fluxing than after 50 pours etc etc. Don Verna

What actually goes on isn't talked about here too much. It ISN'T what is written about either by the vast majority of people here.

[Larry Gibson]

Gentlemen

The RPM threshold has nothing to do with the spin stability of the bullet. It has to do with the effect of the centrafugal force on the imbalances in the bullet during flight. All the discussion on spin stabilization, yaw, pitch and "going to sleep" are completely different aspects. The RPM threshold and what occurs has nothing to do with those.

BTW; if the unbalanced tire was not physically held on to the axle (what it rotates around) it would go bouncing off in some direction. The higher the tires RPM the bigger the bounce would be. The bullet above the RPM threshld is not physically held onto the axis of flight (what it rotates around) and does go off on a tangent or helical arc. The higher the RPM the larger the departure from the axis of flight. It is cantrafugal force acting on imbalances in the tire and bullet that cause both.

Larry Gibson

[45 2.1]

Gentlemen

The RPM threshold has nothing to do with the spin stability of the bullet. It has to do with the effect of the centrafugal force on the imbalances in the bullet during flight. All the discussion on spin stabilization, yaw, pitch and "going to sleep" are completely different aspects. The RPM threshold and what occurs has nothing to do with those.

BTW; if the unbalanced tire was not physically held on to the axle (what it rotates around) it would go bouncing off in some direction. The higher the tires RPM the bigger the bounce would be. The bullet above the RPM threshld is not physically held onto the axis of flight (what it rotates around) and does go off on a tangent or helical arc. The higher the RPM the larger the departure from the axis of flight. It is cantrafugal force acting on imbalances in the tire and bullet that cause both.

Larry Gibson

I believe the correct term is "Centrifugal" you are trying to say, after using two different incorrect versions. It always helps credibility if you can spell the term used or name of the person you're quoting from a book (or not), the proper use of the word and what is actually does.

[Digital Dan]

Williston, Florida — According to Williston Police, a wheel broke free from a car and killed an man early this Monday morning, March 2, 2014.

Police said that it was first believed that a tire compressor had exploded and killed the man, but that later proved to be false.

Investigations revealed that Patrick Woodard of Gainesville was driving north on U.S. 41 in a 1999 Toyota at around 9 a.m. As the vehicle approached the Absolute Auto Body shop at 104 NW 10th Avenue, the left front tire broke free from his car and flew north on the west shoulder of the highway.

The tire then hit a driveway apron with such force, it became airborne. It then cleared a 6-foot-tall metal fence and struck Ivan J. Sandquist, 33, of Williston, killing him instantly, according to police.

After striking Sandquist the tire then hit James E. Fender, 45, of Williston. He was airlifted to UF Health Shands Hospital with serious injuries, according to reports..

Williston Police said they were assisted at the scene by the Levy County Sheriff’s Office crime scene unit and the Florida Highway Patrol.

Police said the investigation into the accident is still ongoing at this time, and they do not have a cause as to why the wheel broke free from the moving car.

[popper]

A similar examination shows that the Magnus force has a destabilizing effect and increases the yaw angle, if its center of pressure is located in front of the CG. Later, this observation will become very important, as we will meet a dynamically unstable bullet, the instability of which is caused by this effect.

Someone in another thread made a comment that we need a tail heavy boolit for accuracy & stability. The quote above is the correct solution. Note the damaged nose bullets will form a helix then take off tangentially. As Larry has tried to state many times, the centripetal/centrifugal forces and strength of the boolit above an RPM value increases the yaw angle and increases the helix orbit. The spin (RPM) and imbalance generate the Magnus force. Directly related to gyro precession. Magnus forces get really nasty in the transonic range.

[Larry Gibson]

Thanks popper. I'm on the road and mostly answer such posts using my "smart" phone which auto corrects what "it" thinks is the word I want to use. Sometime my thumbs mispell words on the small keyboard. Apparently 45 2.1s point is if I mispell a word I don't know what I'm talking about and there for because of a mispelled word the facts are changed. Suppose that's the best he can come up with though. Fact is it is the centripetal/centrifugal force acting on the imbalances that generate the Magnus force just as you say. That I used/mispelled or my phone selected the wrong word does not change the truth of what is occuring no matter how much 45 2.1 wishes it would. Thanks again.

Larry Gibson

[Digital Dan]

As I said, "the baggage that goes with it".

I rather prefer thoughtful inquiry and discussion to sniping minutiae.

Good post Popper, thank you. There's no end to the benefit of rifled barrels, or the evil, if one is a pessimist at heart.

I'm pondering a few odds and ends along the lines of this OP, but my mind has no guard rails. Deforming bullet noses does what it does to illustrate the extreme, but the more profound effect(s) are generated by imbalance about the outer circumference of a bullet. This is so because at a given RPM and due to a greater radius, the effect is more energetic. It does not do to simply file a chunk off the base to demonstrate this because the analysis is confused with the effects of asymmetric pressure (and resulting flow field disruption) on the base as it exits the muzzle.

On another tangent, since this conversation seems to roll that way, it has been commented on several times in recent months that paper patch bullets do not seem to be so subject to these effects until perhaps a higher velocity is achieved? One one part I get it, that being the circumstance where swagged lead bullets are used. Be they lead or jacketed bullets, when they come out of a swage die they are as perfect as machine tools and the dies they make can produce. I differentiate swagged bullets from cast in the context of patched bullets squeaking by the more normal threshold of chaos associated with cast bullets. Anyone have any observations on one or the other?

I have seen some projectiles do some very odd things in my day. One of the easiest ways to see such things is to fire a lot of bullets under different lighting conditions with different and contrasting backdrops. An example would be a minigun being fired with sunlight behind the shooter, toward a cloud shadowed and vegetated hillside. Glimmer is far better for visual observation of a bullet's flight than anything I've seen.

[45 2.1]

The thing to remember here is............. these forces act a lot more on boolits that are tipped to the centerline of the bore when they exit. Whether it is tipped or not is YOUR fault. You loaded the cartridges for your rifle. Make good choices and have the centerline of the boolit in line with the centerline of the bore when it exits and those forces will have very little effect on what your results are.

[geargnasher]

I'm not big on posting photos of high-velocity groups, but this should illustrate 45 2.1's point very well. These two groups were fired from a box-stock Savage 111 Pig Gun in .308 Winchester, 1-in-10" twist, 20" barrel, from bags on a good bench, 100-yard range, at a chronographed 2,347 fps average. Those are 1" pasties. The alloy is water-quenched 50/50 wheel weights and lead soil-stack boots, plus 1% total additional tin. Nothing more than traditional grease-groove lube and Hornady gas checks were used.

While these are not really bragging groups in many circles, and there is considerable room for improvement (the lube, for one thing), I don't believe there are many people here who could match them in such a rifle.

BTW, just for fun I did another experiment with more "mainstream" techniques used by most cast boolit shooters, and with a center aim point got seven out of 10 to strike somewhere on one of the 8-3/4x11-1/2" work order hard cards that I use for most of my targets using the same boolit and powder charge. The difference? All achieved with the way I crafted the ammunition, nothing else.

Gear

ETA I snapped the pic tonight for this post, the groups were fired earlier this year.

[fouronesix]

Oh my, not another rpm threshold debate!

Back to the OP. I think many of those 22rfs are probably leaving the muzzle more or less sonic and probably transitioning subsonic on the way to the target. At those modest vels and given the likely twist of the 22 rf bore- I think most of that helical flight behavior is simply a combination of the basic aerodynamic instability of a common conical bullet shape (the average 22 rf bullet) having it's ubiquitous fight with gyroscopic stabilization. That helical pattern may very well be the precessional phenomenon, as has been suggested, exaggerated with both the center of aerodynamic form and center of mass being off-axis.

[Larry Gibson]

Gear

I'll be driving right by Kerrville in 3-4 days. How about showing me how you do that? I've the time
and will be right there.

Larry Gibson

[C. Latch]

Someone in another thread made a comment that we need a tail heavy boolit for accuracy & stability.

You may be referring to a post I made the other day in another thread concerning handgun bullets. If so.....exactly where was I wrong?

Apologies if I'm assuming too much here.

[C. Latch]

Wish there was some way I could snap my fingers and the rank and file instantly understood the mechanics of spin stability and all the baggage that goes with it. It is quite clear that many don't.

Explain it and you're guaranteed a sticky thread, albeit it one with fifty pages of arguing.

I'd read it, for sure. I posted a thread the other day asking a stability question and got very little response.

[popper]

fouronesix nailed it.
C. Latch -

Magnus force has a destabilizing effect and increases the yaw angle, if its center of pressure is located in front of the CG.

This is a nose heavy config. and is actually the 'drag' configuration.
CG is center of volume. CP (CF) is center of area, for a straight flying boolit. CP moves forward for a yawing boolit. HPing moves CG rearward, no basic change in CP, unless you consider the Paco Kelly 'cup' pressure. I think his approach works in transonic region (22RF).

[DeanWinchester]

Ive got a Keith style SWC 35 cal mold (158g) that I have used in .38 special, .357 mag & 35 remington. It is NOTICEABLY eccentric. The nose is way out from the driving bands. It's right embarrassing to look at. It shoots like a house fire. Out of my 35 remington it groups really well. It's always been a mystery to me, but if it works…...

[C. Latch]

fouronesix nailed it.
C. Latch - This is a nose heavy config. and is actually the 'drag' configuration.
CG is center of volume. CP (CF) is center of area, for a straight flying boolit. CP moves forward for a yawing boolit. HPing moves CG rearward, no basic change in CP, unless you consider the Paco Kelly 'cup' pressure. I think his approach works in transonic region (22RF).

OK, I get all that.....

It still doesn't explain why conventional wisdom (CG behind CP) about spin stabilization is wrong.

[Digital Dan]

With possible exception of double end wad cutters or minie bullets, all conical bullets present CG aft of CP. The larger the displacement between these two centers the faster twist rate must be for a given caliber.

The relationship between gyroscopic stability, bullets and exterior ballistics is a bit difficult to explain in the course of an internet post. There really isn't much there to argue about, the science is clear. The discussion is suitable for books and more than a few have been written.

Modern Exterior Ballistic by Robert McCoy
Rifle Accuracy Facts by Harold Vaughn
Understanding Firearms Ballistics by Robert Rinker

Also, you may take a read or three of Brian Litz online. Some of his work is available via the Sierra web site.

Additional: Dr Mann - The Bullet's Flight, From Powder to Target
General Julian Hatcher - Hatcher's Notebook

[felix]

I do not find ballistics too difficult to understand. Understanding depends upon the teacher's interests as well as that of the receiver's. ... felix