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@freemo the difference is from technological advances in metallurgy and the change from black powder to modern smokeless powders
Modern ammunition designs just don't need anywhere near the same amount of case capacity to generate the same amount (or more) energy.
The .38 vs 9mm is a good example as they have roughly the same diameter bullet but the 9mm has almost ~30% more energy despite being ~30% shorter overall.
He is probably talking about copper jackets. Back in the cowboy days they were just lead slugs.
It's more about the casing, chamber, and barrel of the gun itself, not the bullet.
The Army's most recent iteration of the standard issue rifle (SIG Spear) has a specially designed steel & brass case to help handle the overall pressures that the propellant generates. My understanding is that the new bi-metal construction was needed to ramp up the bullet performance but keep it within the pre-existing overall cartridge length requirements.
@drewfer @freemo Yes, cartridge brass (aka 70/30 or alpha) is great for forming into tubes but isn't very strong.