Pure soft lead deforms very quickly on impact with flesh, much more so than any alloy mix you can shoot. The reason we use pure lead with the PRB is that we are already limited by the powders we use to relatively slow velocities, and because of the poor ballistic's coefficient for all RBs, the velocity drops off very fast outside the muzzle.
You need a soft lead ball to expand on thin skinned game, to increase the size of the primary wound channel, at low velocity, and to shock the vital organs and nervous system to make a clean, quick kill. The pure lead ball does that very well, and is why its such a good " killer" projectile to choose.
Compared to modern bullet technology, and the velocities you can achieve with smokeless powders, the PRB is terrible, " by the numbers.". According to the numbers that last thing you should want to fire at game is a round lead ball.
But, the numbers are designed to define elongated bullets, at velocities well above the speed of sound.
What ultimately matters is the performance of the two projectiles on game, and the soft lead ball will kill game all out of proportion to its weight and kill the animals quickly, inside the normal ranged people can shoot OPEN SIGHTS accurately.
A 177 grain, .50 cal. RB doesn't seem like much of a projectile, but it will kill any whitetail you hit in a vital area, and has done so at distance well beyond 100 yards.
Alloy cast RBS are sometimes used by hunters who want to insure adequate penetration on THICK SKINNED GAME, like Elk, Moose, and Bear. That is a judgment call, where the hunter decides he want to be sure that he puts his Lead Ball deep into the body of the animal he is hunting. To compensate for the lack of expansion he would get using a pure lead ball, he increases the diameter of the bore of the rifle he chooses for hunting this kind of big game. That is why you see the .58 caliber guns show some popularity among experienced hunters who hunt Elk, Moose, and Bear. The larger diameter ball insures a huge primary wound channel, and shooting an alloy lead ball insures that penetration will be very deep on these large animals, even if large bones are struck by the ball.
