The first projectiles fired from barrels were spherical, at least roughly speaking. I say roughly because the manufacturing methods of the time were crude. This period formed the basis of controlling tolerances and quality control in projectile manufacture. People quickly realised that poorly made spheres travelled in unpredictable trajectories. If a gunner cannot predict trajectory, he can only hit a target by luck and not judgement…
The cutting edge of ballistics at that time would have been the guy who could produce the roundest ball! However, balls are not well suited to flying through the air. Air is dense and many of us conducted a very simple experiment demonstrating this as children. You may recall holding your hand out of the car window when young. You would have held the hand palm forward and felt a strong push from the air pressure. Making a fist would have reduced that force greatly (the equivalent of our spherical projectile). Finally, you may have angled your hand to horizontal, fingers pointing forward. The force pushing on your hand is greatly reduced. We have the same size object (a hand) and we are able to control the force that air pressure is applying to the object by dictating the surface area facing airflow. The smallest surface area attracts the least force upon it.
In the early 1800s there were several designs put forward for a cylindro-conical projectile. These were largely disregarded at the time due to the complexity of mass manufacture. However, they clearly represented a better choice in regard to performance. They had a lower front-facing surface area and thus less velocity decay. Maintaining velocity allows for both longer range and higher kinetic energy on target. By the mid 1800s the Minie ball had found widespread use – a lead slug with a pointed front end and a flat base. The Minie ball projectile shape is the basis of all modern small arms projectiles. The designer had realised that surface area was not the only factor in reducing drag; the shape of that surface area also mattered. A pointed shape allows air to flow around it in a consistent manner.
Projectile development continued and the shape remained very similar for many years. Improvements in propellant technology drove material changes. The lead projectile was very dense and easy to manufacture, however it was also soft which meant at higher velocity it had a tendency to tear apart upon leaving the muzzle. As the projectile is spun in the rifling of the barrel, centrifugal force begins to act upon it. Of course, the bore does not allow it to break apart because it is restrained on all its outer surfaces via an interference fit. Once it left the bore it would pull itself apart as it was no longer constrained. The copper jacket was developed to prevent this. Copper was soft enough to travel through a steel bore with a flush interference fit without badly damaging the bore itself. It was also strong enough to contain the lead core against the centrifugal force acting upon it. Lead remained the core component due to its high density and cost. A dense material will carry more weight for its size and thus have more inertia (ability to resist changes in motion). There are definitely denser materials out there but they cost more or are much harder to work with or manufacture. Lead remains an ideal core component to this day.
The last and most relevant major change to projectiles was the development of the hollowpoint boattail design. Previously all copper jacketed rounds had been swaged from tip to base, leaving a cavity at the rear. This seemed sensible at the time as designers postulated that the rear end made no real difference to drag while the tip played a key role. Although that seems to be a good educated guess, it turned out to be wrong. Modern technology allowed us to view airflow around objects. At supersonic speeds the air actually begins flowing around the tip before hitting it. This is due to a band of pressure being pushed in front of the tip constantly. The airflow then swirls around the rear of the projectile in a relatively inconsistent manner, which can induce yaw. Swaging the jacket in reverse leaves a small opening at the tip but closes the base completely. This leads to a lower and more consistent drag. Finally, the rear end was reshaped to incorporate what is known as a boattail. This also aids consistent airflow at the rear and helps reduce yaw or unwanted movement.
That is the final design that most of us will use today. It may have slight tweaks – a plastic tip to give a very slight increase in ballistic coefficient, or changes to the ogive shape. These will give very small performance gains which may be desirable to those shooting high-level competition. There are also many specialist natures which have their own shape to achieve specific performance criteria. Most of these are at the core level a modified HPBT.
Do I see any trend toward further major development? Currently not, other than lead-free options, but even these mimic the shape of HPBT still. We may have reached a point at which we have harnessed all we can in terms of shape and density and development will be in terms of higher velocity. This will be achieved through propellant and firearm design rather than major changes to projectiles.