It's all about FRICTION! Lead bullets are softer than copper, regardless of how hard you cast them - even straight linotype. The softer the projectile, the less friction it produces because it shapes itself to the bore and rifling faster which creates less heat which creates less friction. Although jacketed bullets can be pushed to higher speeds - at higher pressures - due to the protection of the harder copper coating, cast bullets can be launched at near magnum speeds using a crimped on metal base cap and still create lower pressure.
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