Spitzer vs RN Bullet: Stability

[Brians356]

PBS TV has been rerunning old Presidents documentaries this week. Last night they aired an old show on the JFK assassination. Two ballisticians were shooting bullets into different media to help explain the shot which passed through both JFK and Governor Connally.

One of the chaps stated that the long, round-nosed 6.5mm FMJ bullet Oswald fired was more stable in initial flight than a spitzer type bullet, because the former had such a long bearing surface. He asserted that a spitzer, with a relatively short bearing surface, limited to the rear of the bullet, allowed the bullet to wobble ever so slightly in the bore, and therefore would leave the bore wobbling slightly, whereas the RN bullet would not wobble. This was by way of explaining how such a bullet could pass through a human body intact, and nearly un-deformed. The implication was that a spitzer FMJ bullet would be much more likely to upset and tumble inside the body.

One of the show's graphics was an artist's animation of the two bullet types traveling down the bore, depicting the spitzer wobbling slightly (the pointed nose circumscribing a spiral.)

Apart from the lower BC of a round-nosed bullet, one might almost conclude that the RN is more stable, and therefore potentially more accurate over relatively short distances than a spitzer.

Perhaps an unspoken assumption behind the claimed wobble of the spitzer in the bore was that the $20 Mannlicher–Carcano rifle Oswald used would have had an oversize or worn-out bore?

Open for comments.
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[dcali]

Don't know about those two bullets specifically, but in general, long, skinny bullets are more sensitive to aerodynamic jump (which is a primary cause of dispersion), and are therefore inherently less accurate than short squatty bullets, all else equal. It's more complex than this, but its more about the ratios of length to width than bearing surface. There's a reason short range BR shooters use light, short bullets.

 

[Brians356]

There's a reason short range BR shooters use light, short bullets.

Not to divert from this topic completely, but could you give a few examples of the most popular short-range (I assume 100-yd) BR bullets?
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[Auto-X Fil]

Not to divert from this topic completely, but could you give a few examples of the most popular short-range (I assume 100-yd) BR bullets?
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Most people shoot flat-based 6mm bullets in 6PPC, 65-68gr weights. Most top-flight shooters use bullets from small custom bulletmakers that only do BR bullets. Bart's, Bruno, etc. Hottenstein (RIP) 68gr 6mm had a lot of followers. Berger makes a nice selection of 6mm BR bullets that do quite well.

EDIT: that's for group BR. Score shooters use the 30BR and use ~115gr FB bullets from the same sources.

 

[Auto-X Fil]

He asserted that a spitzer, with a relatively short bearing surface, limited to the rear of the bullet, allowed the bullet to wobble ever so slightly in the bore, and therefore would leave the bore wobbling slightly, whereas the RN bullet would not wobble.
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This is why you shouldn't believe what you see on TV - or read on the internet. ;-)

There's no wobble in the bore, unless it's completely shot out, as you said, but I think the whole theory was completely made up.

 

[dcali]

There is such a thing as in bore tipping, and it's bad. Practically, it looks exactly like shooting an imbalanced bullet. Not major wobbling, but nothing is perfect. So long bullets *might* be better at resisting that, but I'd assume that once you hit a certain amount of bearing surface, you've done all you can do to prevent in bore tipping. Once a bullet has tipped, or is imperfect in some way, the short fat bullet will suffer less from it in terms of dispersion.

 

[Brians356]

This is why you shouldn't believe what you see on TV - or read on the internet.

Or necessarily disbelieve something, just because it's on TV or the Internet. The medium is not the message (my apologies, Marshall McLuhan RIP) so what's the point?

(It should be obvious I didn't accept the wobble thesis out of hand, or I wouldn't have framed it as I did, but it's worthy as a topic for comment and discussion.)
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[Brians356]

There is such a thing as in bore tipping, and it's bad. Practically, it looks exactly like shooting an imbalanced bullet. Not major wobbling, but nothing is perfect. So long bullets *might* be better at resisting that, ...

Long bearing surface, in particular, was the assertion. The 6.5mm RN bullet in question might not have been much longer overall than some of the spitzers which replaced it. One could of course construct a very long-ogive bullet with a very short aftward bearing surface, and I have little doubt such a bullet might be subject to yawing in the bore, however slightly, given the tremendous forces acting on the bullet, which I wouldn't expect are perfectly symmetrical, nor would I expect the base of the bullet is perfectly flat and perpendicular to the bore axis.
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[dcali]

I suspect that one of the reasons that barrels get "shot out" is that they wear unevenly/unpredictably. Uneven lands = unbalanced rifling force = tipped bullet = poor accuracy.

 

[Laurie]

Codswallop! (As we say in England.) Bullets' behaviour in air and in animal tissue are two completely different things and whether a bullet yaws / tumbles in flesh is much more to do with its centre of gravity and dynamic stability, than how it behaves after leaving the muzzle in much thinner air where gyroscopic stability is the crucial factor at shorter distances and in supersonic speeds.

Early small calibre smokeless RNFMJ bullets as used in the early British 303 210gn, 155/160gn 6.5s and 173gn 7mm Mauser etc rounds were notoriously poor 'manstoppers', basically as they didn't tumble and created a very narrow wound channel unless they hit a major bone. The British soon found this out in the 'North West Frontier District' (what is now the Pakistan / Afghanistan border regions including the notorious Khyber Pass) when faced with fanatical tribesmen charging their lines and often taking multiple hits from .303 Lee-Metfords and still managing to keep coming. The initial British reaction was the 'Dum Dum' bullet (from the Indian arsenal of that name which loaded it) a crude HP expanding type but that was banned by the Hague Conventions. Western governments soon got around their own rules after the Germans developed the .323" dia. 7.92mm 'S' (Spitzgeschoss) 154gn pointed FMJ in 1904 or so which not only produced much higher muzzle velocities (lighter), much less drag and hence superior exterior ballistics (improved nose shape), but was also found as an added unexpected bonus to be much more destructive on its victim basically because such designs have the c of g further back (are base heavy) and so tumble shortly after encountering a denser medium than air.

We sneaky Brits went further with the 1912 303 Mk VII design which remained the standard rifle bullet until 1957 by making the bullet even more nose light / base-heavy through incorporating a space inside the jacket nose twixt tip and the top of the lead core and filling it with wood pulp.

 

[Brians356]

Good stuff, Laurie. FWIW aluminum was also used inside the tip of a spitzer bullet to encourage upset.

However, the topic here is yawing or tipping inside the bore, not what happens in air or flesh. The assertion made in the show was that the spitzer bullet left the bore wobbling, and therefore would be more likely to upset after initial impact. Your assertion is that the upset is a product of the bullet's shape and center of gravity, which I fully agree with. It seems to me any slight wobble coming out of the bore will be insignificant at the range involved here. My intent was to focus on whether or not a spitzer bullet necessarily wobbles in the bore, not on what effect that might have on terminal ballistics.

But FWIW the show's testing revealed that the 6.5mm RN bullet passed through "JFK" (both gelatin and soap media tested) straight and almost unscathed, but started to yaw just an inch or two before exiting the media. Then, after traversing a few feet of air, it had yawed sideways and created a perfect keyhole entrance on the "Connally" target, in test after test. The autopsy pointed out a curious oblong-shaped entrance hole in Connally's back. The wound channel through Connally was much more destructive than the relatively clean, narrow one through JFK. Of course that was only the first shot on target - the second shot, as we all know, was what truly killed JFK.
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[Laurie]

Ah I see where you're coming from now Brian. I'd be inclined to suspect that the Mannlicher Carcano's (and any other rifle's) barrel condition would have a greater effect on initial wobble in flight than bullet shape, also maybe the way that overtaking powder gasses affect the bullet's alignment immediately after it leaves the muzzle. (Especially back in the days of cleaning barrels with a pull-through string from the muzzle end which quickly produced cord-worn muzzles.)

The claim parallels the old (and much argued about) assertion that modern HPBT bullets, especially VLDs, need 100-200 yards of flight 'to settle down'. I think the consensus amongst ballisticians these days is to rubbish that concept .... but who knows? (I gave up worrying about this years back when I found I could get 0.2-0.4" groups at 100 yards with HPBTs and even the odd one in the 'ones'.)

Incidentally, it is one of these not so well known facts, that the long M-Carcano rifles had barrels with a seriously large gain twist feature (just to show nothing is new and maybe those 19th century Italians stole the idea from somebody's even older muzzleloader design anyway?!) and this 'war-winning' innovation was so important and valuable (!!) that it was one of Italy's best kept military secrets. It was allegedly so secret that not many of the country's ordnance officers knew it (hard to believe as twist rates are hardly hard to check with a cleaning rod or similar!) so that when the 6.5mm long rifles were cut down between the wars, they lost the fast rifling twist section of the barrel and ended up understabilising the long 160gn bullet making them pretty inaccurate.

This always sounded like a shooting version of the sort of urban myths that produce 50 ft crocodiles in sewers and suchlike to me, but maybe the Italians knew this when they cut them down and didn't care a damn anyway as it was much cheaper than fitting new barrels - and they didn't expect to fight any wars against serious armies like the British and US. I always wondered if Lee Harvey Oswald's assassination piece fell into this butchered gain twist category.

 

[Brians356]

I always wondered if Lee Harvey Oswald's assassination piece fell into this butchered gain twist category.

I doubt Oswald's rifle was so modified. According to Wikipedia (so there you have it!) Oswald's M91/38 Carcano was one of the new "short rifles" (originally produced in 1938 in 7.35×51mm and designated Model 38, but in 1940 only chambered in 6.5x52mm and re-designated Modello 91/38.):

The M91/38 rifles were a slightly longer (40.1 inch) short-infantry version of the Carcano TS and had a 20.9 inch barrel rather than the 17.7 inch barrel of the TS "carbine" model.
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[SteveOak]

I am always amazed that when someone asserts something that it is now a plausible topic for consideration.

The basis of the assertion that the spitzer, with a relatively short bearing surface would wobble in the bore.

In order for that to happen the bullet at the bearing surface would have to be smaller than the bore. Keep in mind that the bore is smaller than the groove diameter so this would be a severely undersized bullet.

And this is even a discussion?

Wobble, by definition (Thank you Mirram-Webster) is to move or proceed with an irregular rocking or staggering motion or unsteadily and clumsily from side to side.

The worst that would happen with a severely undersized bullet (such as a 6mm bullet in a 6.5mm barrel) is that it would be cocked to one side and continue to travel down the barrel, rotating about this axis which does not meet the definition of "wobble."

 

[Brians356]

I am always amazed that when someone asserts something that it is now a plausible topic for consideration.

Why does that amaze you? How should a subject properly become a plausible topic for consideration?

What you're asserting, amazingly, is that the totality of internal ballistics is "settled science" and therefore should be off limits for reconsideration and debate. Sound familiar?

In order for that to happen the bullet at the bearing surface would have to be smaller than the bore.

Not strictly true. Your assertion must assume a bullet of infinite rigidity. In fact our bullets are made of malleable lead and copper alloys. It would be even more incredible if such a bullet did not wobble, however slightly, in the bore. My hunch (being only accredited in mathematics but not in physics) is that it would be impossible to prove that such a bullet exhibits absolutely zero wobble - even in a tight bore. If the bullet is malleable enough to obturate under pressure in the bore, it is probably soft enough to wobble - however slightly.

Wobble, by definition (Thank you Mirram-Webster) is to move or proceed with an irregular rocking or staggering motion or unsteadily and clumsily from side to side.

The dictionary definition of "wobble" in the commonest sense is irrelevant in this more focused context. Furthermore, the choice of the term "wobble" was that of the ballistician in the documentary, and one chosen to inform and enlighten the expected lay audience. But I'll humor you and change my choice of terms to suggest the bullet may exhibit "axial precession". Fair enough?
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[Berger.Fan222]

No bullet is perfect and no bore is perfect. Of course the axis of a bullet is not perfectly centered and aligned with a rifle bore as it moves through. The relevant question is "how far off?" are they by some quantitative metric (such as the peak angle between the bullet axis and bore axis).

I've read that bullets in flight "wobble" by a few percent early in flight after leaving the barrel, and this "wobble" as been experimentally verified with various techniques including increased drag and high speed photography. So the question would be how much wobble originates in the bore and how much originates in the transition from bore to flight. An old Hornady manual discusses some of these issues.

Length of the bearing surface probably plays some role, but other factors seem to as well, including the quality (symmetry) of the throat, tolerances of the bore itself, and tolerances and symmetry of the bullet.

I've seen studies that document that the onset of tumbling in the target depends on the yaw angle (between bullet axis and flight direction) at impact. I've also seen both experimental and theoretical work showing that decay of the wobble in flight effects this. There is a discussion in an old Sierra manual on the decay of the wobble in flight and army studies on how the 855 bullet tumbles sooner impacting early in flight and later when it impacts at longer ranges.

The linked report includes Army Research Lab results showing the bullets tested tended to have a max yaw angle of 2-3 degrees early in flight.

http://www.arl.army.mil/arlreports/2010/ARL-TR-5182.pdf

I have often thought that hummer barrels might be barrels that launch bullets with max yaw angles much smaller than one degree, but I've seen precious few of these (haven't we all).

Here's the article (p3) that describes how yaw at impact translates to depth of tumbling after impact: http://www.dtic.mil/dtic/tr/fulltext/u2/a519801.pdf

 

[Brians356]

Incidentally, the PBS "NOVA" episode I referenced in the OP is titled "Cold Case JFK" and can be viewed here:

http://www.pbs.org/video/2365118537/
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[Mozella]

I saw that Nova episode and thought it was pretty good. Keep in mind it was a TV show, not a scientific paper. The challenge, which I think they met quite well, is to present a scientific study in terms that TV watchers could understand AND also be interested in so that viewers don't switch to "The Gay Bachelor Dates A Jihadist" show.

One of the presenters used the term "wobble", for which there is no firm scientific definition. It was the perfect word to describe what he was talking about.

Some might argue that unless the barrel is completely worn out, a bullet can't "wobble" in the bore. I suggest the doubters read one of Bryan Litz's books where he discusses the difference between bullets with a Tangent Ogive and a Secant Ogive and how centering problems with the secant ogive bullets (when contrasted with the tangent ogive form) negatively effects precision.

It makes perfect sense that a bullet like the one which killed Kennedy would proceed down the bore better aligned than most. The very short, round nose of this particular bullet transitions to a bearing surface which is nearly as long as the overall length of the bullet. In other words, it's nearly all bearing surface. The term "ogive" hardly applies at all to this bullet which is shaped more like a piece of re-bar than what we think of when we envision a modern bullet.

 

[Ned Ludd]

The phenomena of projectiles "wobbling" in the bore is called "balloting" and has been known and studied in the context of artillery rounds and other projectiles for some time. It seems that it's not all that uncommon for precision shooting hobbyists to "rediscover" facts that have been known to artillery experts for many years. The definition of "balloting" is on the first page of the first report, along with a number of known causes. The second report involves prediction and analysis of balloting in 8" howitzer barrels.

http://www.prodas.com/XQ/ASP/P.603/QX/Documents/Balloting Analyses during Projectile Development.pdf

http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0923913

It can and does happen, and a relatively short bearing surface may contribute significantly to the effect.