Home Forums Hunting & Shooting Ask The Gunwriters Rotation Speed & Bullet Performance

An article in the March 2004 issue of The Accurate Rifle alleges, on the basis of field experience, that given identical placement on Cape buffalo with 9.3 mm and .375 heavy bullets, bullet rotation speed has a large effect on bullet performance and therefore on the probability of a quick kill. Specifically, the author states that a bullet that is spinning too rapidly is more likely to yaw and then tumble than to penetrate straight. The author says that he has observed that slower spinning bullets, whether solids or suitable expanding bullets such as Nosler Partitions, perform better on Cape buffalo, especially at ranges such as 25 yards and closer. I have no experience and no engineering expertise on which to base comments. But this article (www.theaccuraterifle.com) does raise an interesting issue for those who will hunt Cape buffalo. Comments?

I don't buy that theory in the least. Bullet shape, or more to the point, the shape it's in after upset(if any) is probably more of a factor than anything else. The fact is that twist rates are all relatively slow in relation to revolutions per inch or feet of penetration, and it changes little over great distance to say nothing of 25 yards. The energy used to stabilize a bullet is typically around 1/2 of 1% of total energy carried by the projectile and it is only the amount required to overcome aerodynamic forces, not those of tissue and fluids which are much greater.

DigitalDan, what you wrote makes sense. But the author of that article in the March 2004 issue of The Accurate Rifle claims that a bullet he describes as over-stabilized has substantial yaw during the first yards of its flight. This implies that the bullet's longitudinal axis of form moves at a yaw angle to the bullet's line of flight. The author claims that this yaw angle, at the instant of target impact, leads to bullet tumbling. As I have admitted, I lack relevant field experience. I don't even know how to use simple Newtonian mechanics and elementary differential equations to describe, let alone analyze the physical system. I don't know what the bullet's in-flight yaw angle would be, if it exists, nor do I know anything about possible changes in that yaw angle and the mechanism of those changes. My ignorance far outruns my curiosity.

All bullets have a bit of yaw when they leave the barrel, but the amount is minisule, as in fractions of 1 degree. Faster twists may exacerbate bullet imbalance, but these manifestations are of small import regarding angular momentum and aerodynamic moments. Keep in mind that when a bullet strikes a target, flesh or other, that the medium that is transited renders these issues to insignificance.



There are two primary modes of precession for gyroscopically stabilized projectiles, one fast and the other slow. They coexist initially in the form of an epicyclic precession that resembles a rossette pattern, with the fast cycle making the smaller and more frequent loops about the bullet's yaw axis. Over the course of flight the fast cycle precession typically nulls within 200 yards. The slow cycle responds as a function of gyroscopic stability factor, that is, if the GSF is low the yaw may remain constant, or grow smaller under the influence of a high GSF. If my understanding of these issues is correct, the slow cycle precession is a function of projectile imbalance, the fast cycle one of projectile or aerodynamic jump. In any case, the complete cycle of a single precession occurs over the course of 50 yards or so, and the amount of yaw that is creates is VERY SMALL, even in extreme cases.



Now how this relates to fast twist/slow twist comparisons is somewhat of an 'angels on the head of a pin' discussion. What stabilizes a bullet is angular momentum. Longer bullets require a faster twist for a given caliber, but what is required ultimately is enough stability to overcome the aerodynamic overturning moments that result from the difference between the Center of Aerodynamic Pressure(CP) and Center of Gravity(CG) of the projectile. Larger diameter projectiles generally require slower twists to achieve this stability because the greater radius provides equivalent momentum at a lower angular velocity. So, what does this have to do with your question? The issue is one of stability, and that is one of bullet length and twist rates as required to provide same in the atmosphere. All of these issue pale to insignificance in the density of flesh and bone, and where the speed of sound may exceed 4000 fps. All bets are off. There are shapes that have an excellent history of boring straight through a heavy beast, and they are primarily round nose solids or FMJ's that do not deform. There are shapes notorious for wandering off course such as the boat tail soft points. What is certain is that if the bullet changes shape there are issues at play that are of greater influence than whether the bullet rotates once as opposed to twice as it transits the beasty. Unequal expansion, bone strike, core separation to name a few.



Try this site when you have some time to kill:



http://www.mindspring.com/~ulfhere/ballistics/wounding.html







Try these books for good info on exterior ballistics and other esoteric discussion: "Rifle Accuracy Facts", by Harold Vaughn and "Modern Exterior Ballistics", by Robert McCoy.



Check out Vaughn's book first, and if you find it interesting and are somewhat less than intimidated by math, go for McCoy's tomb. They were real rocket scientists, and their works are available at www.Amazon.com. Hope you don't come looking for me later... <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />

Aside from all the math and theory, there is a widespread belief among African professional hunters that around 2400 fps is the optimum muzzle velocity for shooting big beasties. At this speed expanding bullets don't tend to over-expand. This leaves more shank behind the expanded frontal portion, which helps drive them deeper and straighter. Solid bullets also don't deform--or deflect as easily even if they don't deform.

This doesn't sound as fancy but is derived from at least 100 years of empirical evidence.

JB

Mule Deer: Thank you for stating the consensus of African professional hunters about suitable velocity for large animals. Those folks have actual field experience with more animals engaged than I've ever read about or imagined.
Digital Dan: Thank you for your detailed responses to my posts and for your web and literature citations. I look forward to some interesting and challenging reading.

Also worth noting for American hunters is that there's a very real and surprisingly modest impact-velocity ceiling for effectively killing big game without destroying a lot of "good eatin' meat." There's also a velocity floor. The range of muzzle velocities for this level of effective performance is well under 3,000 ft/sec -- more like 2,000 to 2,500 ft/sec. Nobody in his right mind thinks that any single velocity figure says it all, of course, but the PHs' consensus of 2,400 ft/sec is also a good thumb rule for big critters on this side of the equator.

i would add sectional density to the seemingly magical 2,400 fps formula: an sd of 300 or more for the bullet travelling at that speed.

Those are very sensible numbers and mirror what I've found in my more limited time afield. My ideal muzzle velocities for all my rifles are between 2500 and 2800 FPS, with most averaging 2650 - 2700 (the exception is my super fast .257 Bob <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />). Bullet performance is always perfect and there is very little meat wasted if I can't get a perfect double lung shot. At lower impact velocities the wound channels seem to be more consistant all the way through.

MD, not only from my African experience I agree with your 2400 fps theory. For example I remember the grinning face of my buddy P.H., when the client desparately tried to poke through a big elephant's skull with some brand new ".535 Eargesplitten Loudenboomer" Solids at high speed. We both did it with our plain vanilla .416s and .458s. (I shot right through Lady elephants skulls even with premium .416s softs like A-frames and Barnes Xs)

I agree with your (non)deflecting theory on solids, *if* we are talking "standard", round nose "hunting" solids. And even these deflect now and then (we had a test on this at DEVA institute where .458 RN Solids *did* deflect about the same in the same medium compared to smaller solid bullets.... - "don't shot into the thickets with any bullet!")

Deflecting or tumbling (of solids) is a matter of location of the center of gravity. Just recenty I tested Impala hunting solids from South Africa: Most tumbled after about 15 cm (6 in. in old fashioned British) after impact and left the gelatin bloc. These Impalas of secant ogive shape sport a very long tip.

Of course tumbling of softs in media is a matter of location of center of gravity too - but: A soft bullet changes COG the more quickly, the more its driven fast.

Regards, RD

Roe Deer--

Very interesting results from DEVA, once again.

These days many PH's are using various versions of flat-point solids, which like similar profiles in hard-cast bullets seem to hit harder, AND penetrate straighter, though of course penetration is less than with traditional RN solids. They would of course be slightly shorter than a RN, though whether that would have a significant effect on COG is debatable.

I tried some Trophy Bonded Sledgehammers myself on a water buffalo this winter. These might be termed a "flat round-nose," as its essentially a hemisperhical RN with the top flattened. They feed very well and hit pretty darn hard, especially at 2350 fps from my .416 Rigby.

My PH in Botswana last year was using some flat-nose, secant ogive 475-grain solids from some South African firm in his .458. Can't remember the name but he really liked them, said they hit hard and penetrated straight.

No matter how hard we try, there still seems to be a happy medium.

Good hunting,
JB