While I love the idea of farming some spawners and essentially having them feed their own demise, the bone bullet drums fall off just slightly slower than regular magazines in terms of usefulness.
Can you add more tiers for bone ammo?
I'm thinking a T2 version similar to piercing, maybe if you want to stay cheap simply harden a T1 via furnace with a medium time on it so symbolize a controlled heat and a T3 with is simply two U-258 and a T2.
That way the bone drums would be an all around better alternative to regular bullets, but require biters to be killed instead of just mineable resources.