Cast Bullet Shooting 400 series: I did some extensive experimentation (over about 7 years) in producing soft nosed, cast bullets. In a nutshell, bullets made common wheel weights and linotype (9:1) were cast, cast sized and gas checked, then heat treated. Normally in the heat treating process I would quench the bullets in room temperature water and let them dry before lubing and loading. When the bug to anneal the nose of the bullet came on me, I made a pan that held 80, 30-ish caliber bullets. The pan was made from two biscuit sheets: the upper one drilled for the bullets and separated from the lower pan by 5/16" spacers. I took a plate of aluminum and drilled it with the came hole pattern as the pan. It was 2" thick and the same length and width of the pan holding the bullets.
In use, the bullet were placed in the pan for heat treating and the aluminum block placed over the noses of the bullets. The entire assembly was placed in an oven at roughly 460F and left for three hours, at which time it was removed and set a pan of water @ about 3/16" depth. A small fan drew away initial condensation and the aluminum plate was covered with insulative material to retain heat. It was left until it was only warm to the touch. The bullets were then removed and dried, lubed and loaded.
It was a lot of trial and error. Shooting into compressed bales wet newspaper I got some reasonable results. .301" noses would expand to and average of 33 caliber. Occasionally I got one that would not expand much at all, being hard thru and through. I blamed this on initial steam vapor from the quench getting to the nose. I placed a small pump so that cool water could be circulated as the aluminum block did its work. All mostly worked as planned, to various degrees.
Finally it came to me that heat treated bullets were at a Bhn of about 31. That was about 1/3 the hardiness of copper. A 175 grain 30 cal bullet, fully heat treated and moving 2500 fps from a 308 killed mule deer handily. I stopped trying to soften noses.
FWIW I did try the dual alloy pour. Never worked satisfactorily. I got shears. ~Muir