Does anyone else ponder the narrow range of projectile diameters?

Very interesting. I was thinking of something along the same lines with barrel length. Short barrel = low velocity , longer = faster. What happens at the extreme ? How about a 50" barrel, 100", 50ft.? At what point is maximum achieved ? Lets us a standard .223 for reference.

No, not really. 150 years ago, the only real way to increase lethality- given the limitations of black powder- was to increase bore diameter. With pretty much everything humming along at a rather sedate velocity, even the big magnums were tolerable to shoot. But, once smokeless powder came on to the seen, velocities skyrocketed and more could be done with less. But as muzzle velocities increase, so does recoil. Most people will tell you that a 50 BMG is more fun to let others shoot, than to shoot yourself. A big bore rifle, retaining smokeless muzzle velocities is either a dread to shoot, a drag to carry, or both. There’s not many people going out and shooting 577 TRex for fun. And those that are, have a twisted definition of fun.

A 30 cal smokeless gun surpasses all but the biggest BP thumpers. And to attest to that; Walter “Karamojo” Bell killed 800 elephants (out of a total of 1,011) with a 7x57 Mauser.