I have wondered that also ,i cast bullets for my Sharps .1886 ,and other guns but i wanted to cast for an M1A.
Only one semi rifle will do this and i have done it ,it is the H&K 91 ,or any of its clones.
It is because they are blow back operated.
I even wondered about wheel weights because they are harder ,but i have heard don't do it.
How about Zinc? the BP cannon guys use it for balls .