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It occurred to me that these cast Zamak bullets would be ideal also for the 5.7. If there are 7000 grains per lb, a 28grain Zamak bullet would cost a penny each. This seems like it would be a great practice projectile that could be driven at comparable velocities to our favorite 28gr defensive loads for similar point of impact training.
Going one step further, a custom mold could be created with a "Spoon Tip" front like the HK CETME cartridge, that would allow the bullet to tumble. This would make it useful for both plinking and defense.

It seems like Zinc bullets have been used in the past. Dillion used to market them under the "ZEE" name brand, and there was some experimentation in the late 40's and 50's with them as well.
I think the real issue is creating a mold with bands to reduce friction and keep pressures safe. Are there any other issues you guys see with the idea?