The posts made so far have got me thinking about what would be the process involved if I wanted to make a new weapon/bullet; how would I go about it, and what would be the process?
I freely admit that I know nothing at all about bullet or weapon design, so all I have to offer is my beer and peanuts thinking.
In designing a new rifle/ammo, my first consideration would be a lethal range and accuracy specification. For example, I would specify 100% kill at 500 yards, with a 6 inch group.
The kill specification would require that a specific energy be delivered on target by the impacting bullet. Energy delivered is a function of mass and velocity.
So, to kill the target, the bullet must have a minimum velocity of V1 and mass of M1.
Accuracy is determined by the bullet’s velocity, mass, and its rate of rotation about its longitudinal axis; the rate of rotation is a function of the rifling and the length of the barrel.
However, in order to impart the original spin, and maintain in flight stability, it seems that there would be a minimum length for the bullet; say L1.
The practical maximum length of a squad weapon barrel is say L2 inches, and there is a corresponding upper limit to its weight, say W1 lbs.
So, we have a bullet of length L1, a barrel of length L2 and barrel maximum weight W1.
The specific propellant to be used in the cartridges would be capable of producing a maximum pressure of P1 per unit weight. This, and the barrel specifications, would put an upper limit on the total propellant charge of the cartridge.
Given the design envelope for the barrel, all of these considerations would put a lower limit on barrel wall thickness, and a consequent maximum on the bore diameter, call it Bd.
So, at this point, we have bullet length L1, a barrel length of L2, a barrel weight of W1, and a maximum bore diameter Bd.
We then construct a bullet out of normal materials (copper, lead and zinc) of length L1, diameter Bd and weight X; “X” would be a function of L1 and Bd.
Then, using the maximum propellant charge, we conduct a field test of the barrel and cartridge combination; and measure the impact energy, and accuracy, delivered on target at the specified range.
If the delivered energy is more than required by the specification we can cut back on the propellant. If the energy is insufficient, then we can cut back on the diameter of the bullet.
If the accuracy is off, then we can modify either the length of the bullet, or the barrel rifling.
So, starting with the original kill specification, we can modify the propellant charge, the bullet diameter, or the bullet length in order to meet the spec.
By this process, we end up with a bullet caliber that meets the kill specification; but does not necessarily correspond to any “logical” numerical value.
This line of thinking may explain why we have such odd bullet calibers; the bullet caliber is the result of a process directed at meeting a specification.