Like the design of supersonic fighter jets, bullet shape will always be a compromise. The optimum wing and fuselage design for a Mach 2+ fighter is old established knowledge but the unavoidable bottom line is that it will spend 90% of its airborne time at subsonic speeds. The very low speed for take-off and landing already determines that a perfect supersonic wing design will stall on getting airborne so we do not see any fighter with true supersonic wing designs.
Similarly, despite a hunting bullet spending 100% of its flight time above Mach 2 the object of its existence is not flight but impacting and penetrating very dense bone and meat to mechanically cut through the animal’s heart and sever the oxygen flow to the brain so that neurological function is lost.
This section is about the special bullet shape needed for special performance. Special performance when killing thick skinned, heavy boned, densely muscled dangerous game relates to penetration through these obstructions - and ONLY that. For the very thick skinned Cape buffalo and elephant the importance of a high BC bullet is so low on the priority scale that it can be ignored. Nobody shoots an elephant or a buffalo or even a giraffe further than 50 yards. Special performance for a special need.
Bullet manufacturers in the USA have for scores of years defined the ”special performance” regime for hunting bullets as their low drag ability in flight. This, by marketing spin has re-actively become the #1 priority for most hunters. In South Africa the highest priority for most hunters is the terminal moment, so their demand on bullet manufacturers is to define the region for special performance as that 1,000th of a second after impact. The same reason why heavier, slower bullets are in demand here.
Bullet design in the USA is manufacturer and marketing-spin driven, while in Africa it is driven by hunter requirements for on-game performance - one shot through the heart killing ability. Bullet manufacturers respond to user demands here or they will go out of business.
The best shapes for lowest aerodynamic drag at supersonic velocity cannot be applied due to the requirements of the special performance demand mentioned above (compare Figure1).
Similarly, the requirements for low drag at subsonic speed that at first sight appears to be better suited to the special performance demand can also not be utilised because the massive supersonic drag on these designs causes unacceptable bullet slow down even in the first 50 yards (see Figure 2). Popular values for ballistic coefficients (BC) - which often do not follow actual supersonic aerodynamic drag principles, -are useless when a DG bullet is designed to meet its post impact special performance demands.
Figure 1. ¾ Parabolic Cone.
This is the best aerodynamic shape for Mach 2 - the speed of a 500 gr .458 Lott at 50 yards. Low shock wave drag is the design driver for the ¾ Parabolic Conical nose section. For in-animal performance no poorer shape can be imagined.
Field experience has long ago showed that a slender nose, also being too long for magazine rifles, gets displaced (yawed) in virtually every instance by bone in buffalo or elephant. The centre of mass of such bullets are also too far to the rear. The biggest flaw of such a design is the massive in-flesh surface friction it experiences due to its into-tissue “creeping” which causes linear and rotational friction, slow-down, and loss of gyroscopic stability, wobble, and low penetration. Furthermore, slender nose shapes are too weak for the impulse counter-force they must endure at impact.
Comparing bullets to jet fighters again: the only advantage this shape has for a Mach 2 jet fighter over a straight-edged cone is the lack of a shock wave where the cone flows into the fuselage; a shock cone here would cause unacceptable conditions for some systems, including the perspex canopy. On a bullet inside an animal with 800x the density of air in such a shock front from a cone shape angular transition from nose cone to shank is very beneficial.
Figure 2. Elliptical.
This nose shape is the best design for lowest drag for velocities below 850 ft/sec. For any hunting bullet it is an aerodynamic drag disaster at supersonic speeds as its critical drag rise Mach is already at M=0.80. Even a flat nose, conical shape has lesser shock wave drag at Mach 2. The velocity degradation on a bullet of such a nose shape is acute during the highest velocity regime which is out to 50 yards from the muzzle.
This shape does put more mass into the front end of the bullet, making it stronger, but the long nose still is its Achilles Heel, so to speak, regarding in-tissue deflection - particularly by bone, causing yaw, followed by wobbling and immediate loss of penetration.
Figure 3. Elliptically blunted cone.
This blunt design was a feature of a Winchester FMJ (nickel coated) in the 1970s. It had stable penetration in soft skinned game due to the almost flat nose but the ogive design still caused it to slip into the animal tissue which caused wound channels of less than calibre size.
This was the start for moving away from aerodynamic performance towards animal tissue performance. It was popular on thin skinned animals like eland due to the low amount of meat damage. It was effective on elephant frontal and side-on brain shots but not on heart shots on either elephant or Cape buffalo.
Figure 4. Power Series.
The midrange shape in the radii indicates the mean compromise between a straight cone and the elliptically blunted cone. It retains the low supersonic drag coefficient of the straight cone while allowing for a blunt nose to create fluid containing tissue to shear ahead of the bullet at the moment of imminent contact, preventing the onset of friction drag on the bullet immediately. This was the start of the cavitation principle.
With a larger blunter nose and therefor flatter conical angle of attack this is close to what the Peregrine VRG-2/3 series have.
Figure 5. Bi-conical.
It was previously stated that a distinct shock front immediately in front of a flat nose bullet creates a tissue shearing action and the start of a cavitation bubble. Another shock front at the transition from nose cone to shank enhances the cavitation bubble and lessens skin friction drag tremendously.
This is an acceptable and cheap rocket design. By removing the frontal cone at Ø1 the start of the perfect bullet for in-animal special performance has begun. As mentioned earlier bullet design is a compromise between aerodynamic and in-animal performance, and for the best performance to kill a Cape buffalo at 40-50 yards aerodynamics must take a back seat.
Figure 9. Super Cavitation.
This concept will immediately be understood by boat owners: when a propeller cavitates in the water the removal of the drag force on the blades cause an immediate and significant increase in rpm (angular velocity). For a bullet inside animal tissue this ability has huge benefits.
Very promising empirical results on elephant have been obtained with this concept in South Africa. The flow separator is a separate disk centered on the mephlat which strengthens a shock front around the body by vapourising any fluid containing tissue. Penetration through tissue and bone is impressive.
The main advantage is preventing friction-creating tissue to be in contact with the bullet shank. Keeping this - what is known in aerodynamic terms as the "wetted area" as small as possible, skin friction drag is limited to the flow separator and penetration is considerably enhanced per impulse force. The flat nose concept of the Peregrine and GS Custom bullets achieve this to a certain extent - not only inside an animal but they also have considerably less aerodynamic drag at Mach 2 than round nose dangerous game bullets.
(Next: Cavitation and Super Cavitation bullets)